Which part? Win rates are kind of misleading, as can be any statistic- say you play one game and you win, you have a 100% win rate, good job, go tell all your friends but conveniently leave out the part where it's skewed by lack of data. Of course, there's no way that's the case here, but the same rule applies on a larger scale. Sometimes lesser-played champions have a higher win rate simply because the few who play them know what they're doing. Less opportunities for losses in that scenario as well.
As for the nerf, it's just to his E, dropping the ratio and base damage by a bit; and in the case of Fiora, yeah, I know, it's sad since I really like her. Unfortunately melee carries as a whole aren't very competitively viable, so they're kind of automatically less popular.