Generally, should also be considered that, due to the general tendency for players to pick damage dealers, any champion that doesn't do very much damage, yet nonetheless improves the overall team composition, should naturally sit above 50% win rate.
Tanks are not played as often, but a team with a tank should perform more capably that a team without a tank, especially at the low to mid levels of play. Since the majority of the time, players don't play tanks, a player who does play tanks should improve his teams win rate. Similar to enchanting or peeling supports, who don't do damage, don't bring engage, generally aren't played as often as "playmaking" supports.
This is why, IMO, it's okay for Nami to sit on 53/54, while less OK for Ahri to sit on 53/54 (though I still think it's OK for Ahri to have a high winrate b/c her winrate is a symptom of the game being increasingly reliant on one-shotting people instead of actual strategy, and Ahri is very safe).