Not only are champs op or up at some time, but, for example, a team that doesn't ban Fiora probably doesn't know that much about the meta, thus if the other team picks her and wins, it is partly because the other team might be less knowledgeable about the meta. The same thing goes for up champs, but in reverse, as hose who pick an off meta champ might not have the game knowledge to know that they are weak.
Just a theory
I think the game knowledge aspect is very important.
Like just because a champion has a high pick rate and high win rate doesn't necessarily mean they're that OP.
A champion could be an 8% pick and 52% win rate, strong but nothing overpowered or deserving of a nerf. Then the good players with good game knowledge catch on that this champion is pretty good / meta right now. The champion gets picked more (pick rate up to 20%-30%) and these players, given that they're paying more attention to what's strong, are probably better at the game overall and likely have an over 50% win rate themselves (say 54%). So that compounds on the champion and you get a 30% pick rate with a 54% win rate coming through and the champion looks really OP.
