Many champions that are being buffed for the worlds patch already have an EXTREMELY HIGH WIN RATE, like riven.
Others aren't OP but more of a counter pick or a multiple lane champions are getting NERFED because Riot doesn't like multiple lane champions for some reason and really wants to nerf them into oblivion.
Example. Karma and Sylas aren't at all OP and don't have an outstanding win rate at all. Riot has been trying to murder sylas jungle for a while now for what reason? Karma is known for her Aggressive nature and her fair CC and is very flexible to sup, top , mid , and even sometimes BOT. But Riot is like hEy Lets get rid of karmas only source of damage so we can kick her out of every role because she now has no damage and make her not an aggressive support anymore because mUlTiiple rOleS aRe bAD. So these champions are gutted from their role hmmm I think Fiora needs another buff, Oh riven has a 54% winrate but isn't played in pro play