High Champion Mastery is not a forecast for success rate
I've been noticing in the games I've been playing where I've played champs in which I have high Champ Mastery levels (5 to 7 depending on the champ), I'm getting judged by my teammates as a "Fake", "Boosted" and other derogatory terms when I perform poorly as those champions. Look, Champ Mastery, in MY point of view anyways, is not a prediction on how well a player will perform, when a high mastery level is seen in the loading screen, or the Emote is flashed. As much as one might wish it, the stars don't ALWAYS perfectly align for an excellent game. Those with LVL 7 Mastery levels arean't ALWAYS going to to 10/0 or some other amazing performance. They're only human, and they make mistakes just as much as any other player.
Perhaps a matchup just isn't in our favor.
Perhaps, no matter how one tries, stress or worries from outside source just can't be fully put away for a game.
There are countless more reasons I could go on with for justifying a poor performance despite one having high Champ Mastery on a character in the game. Now, on the other hand, there are times when a poor performance CANNOT be justified.
Perhaps the player is trolling.
They're just in a bad mood and want to ruin the experience for everyone else.
Just like what I said above, there are countless reasons or explanations one could give for this too.
I suppose what I'm trying to say here is exactly what I say in the title above: High Champion Mastery is NOT a forecast for success.
An LVL7 
could go 10/0 one game, and 0/10 the next.
An LVL7 
could land every skillshot with pinpoint precision one game and not hit the broad side of a barn in the next.
An LVL7 
could time his R perfectly to win every teamfight one game, and not save even himself with it the next.
Granted, the likelihood of these events aren't HIGH, but they AREN'T IMPOSSIBLE either.
Champion Mastery is not a prediction of a player's success rate; it is a measurement of their knowledge of that champion's mechanics, IMHO.
