Depending on which generation we're talking about, I can either guarantee that the other 1/4 used either RNGed 'mons or hacked 'mons that just weren't obvious (legitimate locations, etc.), or I can say that the 3/4 who did really didn't want to put in any effort at all. Prior to Gen VI, you were either RNGing or hacking if you wanted competitive 'mons (I guarantee you every VGC 2013 Worlds competitor used either RNGed Pokemon or hacked Pokemon capable of beating hack checks which wasn't hard to do), but Gen VI onwards, getting competitive Pokemon legitimately is so easy that there's really no excuse to be cheating. I personally have no issue facing hacked Pokemon as long as they can theoretically be obtained legally in game (which means legal moves/IVs/Pokeball etc.) and the person using them declares them as such, but this is talking in a Gen V or earlier context. However, I can understand why others might not be so comfortable with it (someone could dump months into one team only for it to get steamrolled by something someone put together in 5 minutes, etc.)
That being said, I'm friends with several admins of Streetpass groups in San Diego. They want to have a Pokemon League with the Gen I Virtual Console games, but they're thinking they'll have to allow Rare Candy cloning through Missingno glitch if they do that as there's no other good way to get your Pokemon to level 100 and there's no option to de-level everyone's Pokemon to level 50. In general, there's a legitimate reason for allowing certain glitches. It makes you kind of glad that Game Freak's made the competitive side of Pokemon more accessible without cheating/glitches in recent years.