Why I think Champ Master sometimes causes incorrect assumptions
To start off, I am an unranked player (Was a Silver 2 but lost the spark and mainly stick to Norms now, with the occasional ARAM and Rotating Game mode as they catch my fancy) and my champion mastery levels at 5 and above are as follows:
L7
L6 with a L7 in the works
L5 with a L6 in the works
These ones are L4's or lower (some closer to L5 than others), with many more that I choose not to put here so that it doesn't become too overcrowded.
What I'm trying to show with this is that I try to have a diverse champion pool, with those that I prefer to play and those who are backups for when my mains are not viable. Thus, I have varying levels of Champ Mastery with said characters.
Now, let's move to a hypothetical situation. Let's pretend I queue up for a game. Let's say I end up in Top (as that is one of my preferred positions, but I'm willing to play anything if needed to switch). Being Top lane, (and assuming he makes it through the ban phase were this a Ranked or Norms Draft) I choose . Everything proceeds as normal, and we make it to the loading screen. However, along the way, I failed to notice that the enemy team picked a hard counter to my . (Say, 
for example) In the loading screen, my teammates see my L7 token, and think "Ahh, this guy's an L7 . Being that good means there's no way he can lose to that , right?" Well, we get into our hypothetical game, and despite my best efforts, I still lose to this . We proceed to lose the game (Perhaps due to my loss, the is allowed to roam and thus eliminate any leads we had in other lanes, or other varying circumstances) In the post-game chat, my team proceeds to flame me for losing to despite being "A L7 Darius".
This hypothetical situation gives us an example of unjust assumptions made by a team due to the high Champ Mastery level of one of its members.
**POINT 1: High Champion Mastery does not always guarantee success **
When a player has high levels of Champion Mastery on a given character, it can often be assumed that they know what they're doing with said character. Most of the time, this is true. But sometimes a player is granted Champion Mastery points based on victories with that character, simply because they were on the winning team, not because of their performance in the game itself. A player could have an L5 token, and yet not land a single hook all game because their actual performance with him does not reflect their shown Mastery level. They got their L5 token by simply being on winning teams when their own performances only warranted a C grade, for example. A win boosts the amount of Champion Mastery points one gets simply because they won the game, not because they performed well.
POINT 2: Getting Counter-Picked can throw a wrench in even a high-mastered player's plans
Getting counter-picked is never fun. Let's pretend you're in Champion Select. Your main got through the ban phase, and you even got your preferred lane/position with that champ. The time comes, you select them and lock in. Then a player on the enemy team picks and locks in a hard counter to your main. Infuriating, isn't it? You got your main, even in your favorite position, and now your performance is going to take a hit because you're going up against a hard counter to them. Even if they don't do well in-game, your performance still isn't at its A-Game, simply because you were counter-picked.
It's the same for a player who has even L7 tokens on their mains. You know what you're doing with them, you might even know some ways to somewhat keep up despite the counter-pick. But slip-ups and mistakes, no matter how rare and watched-out for, are still possible. On top of that, the feelings of anger or annoyance at being counter-picked can also hinder your performance. And that brings us to the third point.
POINT 3: Current Emotional/Physical status can alter one's performance even when one has high Champion Mastery
Being only a single being, I can never know what another person is going through at any given time (Unless I'm a creeper/stalker weirdo, which I am NOT and I hope to never become one). I don't know what another player is dealing with at any given moment when something may go wrong in-game. Say the power goes out at their house, or they lose Internet Connection due to one reason or another, or an unexpected emergency happens that demands their full attention or any number of other things happen. I'll never know, because I'm not them and they may not be able to get to the keyboard (Or other forms of communication, like Skype) in time to tell me. Say they DO make it back and get back in-game, but their performance suffers because of the things currently going through their minds. You can't help it; things happen!
If you've read this far, thank you for reading my thoughts on this matter. I showed you some of my Champion Mastery levels, and with the hypothetical situation I gave, you can't win them all. So please try to keep a realistic view of a player's performance regardless of their mastery levels, because the 3 points I gave or any number of other factors can alter it.
GL and HF in your games. [slayer-pantheon-thumbs]