I wonder how much of this is a vicious cycle. Everyone knows that 3.5 is hugely popular, so they label their games as 3.5 even if that's not what they're playing, so 3.5 shows up as hugely popular in the rankings. This isn't theoretical; looking for a 3.5 game over the last few days, I've come across several 5e games (and one that had nothing to do with D&D/D20 at all, but stated in the campaign description that it was labeled as such to draw attention) all labeled as 3.5. I don't have a clue how to figure out how much skew this introduces, it's really just idle curiosity.