To be clear, those are the +Def/-SpDef and -Def/+SpDef natures. If you have a fast mon with a recovery move, a physical attack, and a special attack, PS will often recommend Lax or Gentle as the nature. This is basically always incorrect, and these natures are not viable on any mon in any tier that I'm aware of. It will give -Atk/Speed if the physical move is something like Rapid Spin, but some utility physical attacks like Knock Off and U-turn are treated as real attacks and the sim therefore avoids giving -Atk natures if these moves are on the set. Perhaps sets like this should get -Speed by default?
An example: Mew with Knock Off/Psychic/Thunder Wave/Recover will get a suggested Gentle nature, when it should really get -Atk.
An example: Mew with Knock Off/Psychic/Thunder Wave/Recover will get a suggested Gentle nature, when it should really get -Atk.