Really? Well I don't play ranked, but in normals and ARAM i have observed something approaching a negative correlation between champion mastery and performance on a champion. People with champion mastery 7 on a champion are often unbelievably awful at playing that champion, really mind blowingly bad, and people with champion mastery below 4 often do very well. I think it is because champion mastery has little at all to do with how good you are at the game, at best it indicates you have played a lot of games, but many people play a lot of games without ever rising above a low level of ability.