Because the sample size is so ridiculously small it's next to irrelevant?
Also the fact that almost literally every champion played in those ELOs have a statistical winrate of around 54-58% winrate, far inflated above their normal statistics? Or are either on the complete opposite side and are like 44% winrate because they're champions like Diana who are garbage-tier regardless?
It's about potential.
Lee Sin is still near one of the most frequently picked champions in Master/Challenger. He also has a negative winrate in both on average. It's not because he's OP. It's because he has the potential to make a teamfight changing play if done well.
Jayce with like 52.8% in Master, 49.7% in Challenger. Fairly large number of games played (relative) in both.