Poison resisting Fighting: This one even I have a hard time thinking of a concrete theory to. The basic thought behind it is that "poison would weaken/corrupt the Fighting-types muscle strength of chi energy". But that would imply the Pokemon using the Fighting-type move was already poisoned or somehow the Poison-type emits a "poison aura" that once you get close to it, if you're using a Fighting-type move, it'll be weaken. Also why wouldn't Normal-types be effected by? Somehow a Pokemon more trained in controlling their muscles and chi energy is MORE susceptible to getting it weakened compared to an untrained Normal-type? And there's nothing about the Poison-type that would indicate it's somehow able to take punches and kicks to the face better than any other Pokemon. Even if they have poison on their skin/scales it wouldn't act fast enough to weaken the attack as soon as contact was made (and inertia alone would bypass that); maybe acid but that's very loosely tied to Poison-types.
Flying super effective to Bug: "Birds eat bugs". Except not all Flying-types are birds. And even if it is the bird hitting them with wind isn't eating them. And if eating them is the only justification then wouldn't that be a Pokemon-by-Pokemon case depending if it's based on an animal that's an insectivore? Only other theory I could think is that, due to Bugs having an exoskeleton instead of a normal skeleton makes them more rigid so a strong wind could more easily knock them about helplessly. But if that's the case why isn't Flying super effective against Rock, rocks are known to be eroded by wind just as much by water and plant life. "Bugs have squishy insides and rocks don't". Fair enough, but this gets my point across that we had to go through this whole chain of logic to get to that point. Flying is super effective against Bug because the exoskeletons make their bodies too rigid to avoid being flung about by the wind energy which would cause their squishy insides to be more easily injured.
Electric resisting Flying: Okay, sure, lightning may be uneffected by winds, but once again we're dealing with a living creature that just shoots electricity, they're not made of the stuff (usually). And I couldn't find anything that electricity could be used to disperse wind, anytime I type in "wind" and "electricity" it comes up with stuff about wind power. Okay, let's figure it out for ourselves then. We know electricity superheats air because that's what creates thunder, it's the sound of the air molecules rapidly expanding out due to all the heat. So I guess if the Electric-type is quick enough they could disperse the winds by superheating the air with their electricity ripping apart the wind current. But this would mean every time a Flying-type move is used on an Electric-type they instinctively discharge fast enough to weaken the damage from the wind energy. But then we get to another issue: what about all the physical Flying-type moves? Pecking, attacking with wings or acrobatics, do we just assume that they're also surrounded by a veil of wind energy which what makes them stronger than normal but Electricity also disperses that veil?
Flying immune to Ground: A classic example since it the reasoning only makes sense for 1. Flying-types that aren't touching the ground and 2. the moves Earthquake, Magnitude, Bulldoze, & Fissure. But otherwise this doesn't make sense. We have plenty of Flying-types who don't fly (Farfetch'd, Dodrio family) or stay mostly on the ground (many early stage bird Pokemon, Scyther) they'd surely be effected by the ground shaking. And we have plenty of Ground-type moves where the user is either throwing ground or just launching an attack that's infused with ground energy (there's even some moves which is ground energy blasting up from below). And it can't be just that Flying-types are just that good at dodging because they're weak against Rock-type moves which is the same idea but rocks are harder than packed dirt I guess (though that doesn't explain the bone-based ground-type moves).
Bug resisting Ground: Like, I get many bugs make their home in dirt, but would that really protect them if an earthquake causes the dirt to close up around them? Or a hard object hits them charged with ground energy? A Normal-type move hitting Bug causes neutral damage, but the bone moves, Drill Run, High Horsepower, or Stomping Tantrum somehow doesn't do much more damage because ground energy. Like, what is even with the ground energy the Bug-type is resisting even? If the move uses physical dirt, fine, bugs know how to break that apart; but these are just normal attacks infused with the energy from the earth, if you take neutral damage from Normal-types moves it's either only going to hurt as much or even more. I guess you can say their exoskeletons just have a special anti-ground property to it but that's just giving up at that point.
Grass resisting Ground: Earthquakes are just as devastating to plants as they are to everything else. And a plant swallowed up by the earth is going to die without the sun to get energy to do photosynthesis. Plants may root themselves to the earth and in some way erode them, but that only means when the earth those roots are spread in start shifting they're more susceptible. "Maybe the Grass-type roots are keeping the ground together". I was being metaphorical, unless the Grass-type is physically connected to the user, this is about a Ground-type move being used on plant creature. How does the plan creature take less damage from hardened earth, bone, or just a move charged with earth energy?
Fighting resisting Rock: I can SORT of see the reasoning, but still it's either a hard material or another form of earth energy being used on the Fighting-type. Sometimes well-tuned muscles aren't good enough to shatter it... because then you gotta ask why Ground-type moves hurt it as they've sort of established ground is softer than Rock. OR how an untrained in martial combat Normal-type does neutral damage.
Fighting/Flying/Poison/Ghost/Fairy resisting Bug: Like, why? What about a bug bite/sting or "bug energy" are these Types able to be taking less damage from yet other Types they receive neutral damage from? For Fighting and Poison you can read some of my thoughts above about them resisting Rock and Fighting, respectively, and apply the same arguments with Bug. Flying creature may predicate on bugs and Fairy are higher in the natural order but does that mean their bites and stings hurt less to them? And Ghost has the opposite problem, they're immune to Normal and Fighting for good reasons yet bug energy somehow make their physical moves make contact?
Flying/Poison/Bug resisting Grass: Once again, just because flying creatures and bugs eat plant matter doesn't mean they're exactly resilient to tricks they may have. Some plant species have grown thorns to make it difficult for flying creatures and bugs to feast on them. Heck, carnivorous plants EAT flying creatures and bugs. Focusing on bugs, Grass-types also have the fungus grouped with them and there's plenty of fungus that take control of bugs minds. As for Poison, sure they hurt plantlife but at the same time plantlife can have what's needed to make medicine to cure poisons, so it's a sort of two-way street.
Grass resisting Electric: I get the idea, plant life can have electricity flow through them, down their roots, and harmlessly absorb it into the ground. But let's not pretend that lightning has caused not forest fires or split a tree in half. It's still superheated plasma which causes the air around it to expand happening all in a microsecond, that's gonna cause some damage.