If a champion is popular, yet somewhat difficult to play, they will often be broken beyond belief, yet have a low winrate due to people who are new to them dragging it down.

High banrate typically also drags winrate down, because a champion with high banrate will more often play against counterpicks (people playing counters won’t ban it). So a champion like Zed who permanently dominates ban charts might be below 50% and still be completely broken (not saying he is now, but there were such seasons in the past).

Yone would currently be a good example of a champion who is extremely overtuned, but due to very high banrate and high popularity combined with high difficulty his winrate looks kinda dog.

On the other hand, champions like Rammus who are typically picked only to counter specific team comps might be perfectly balanced and yet have high winrates