You are not understanding me.
If you flip a coin enough times, since your selection is random (you have no way of consciously influencing which way it lands), you will approach the natural distribution of 50/50 for heads/tails respectively. This is quite intuitive: if you flip 1000 coins you will likely be very close to 500 heads and 500 tails, again assuming the coin is equally weighted, giving an equal probability of landing on either side. This phenomenon is called the Central Limit Theorem and is essential to understanding basic statistics:
Likewise, in your game you have 11 other players in any given game. We can assume these players are independent of eachother in relation to you, and we can assume they are random. The skill of the players do not affect eachother from one game to another, each game is a clean slate .You agree up until here, right?
Okay, so the theory here is that if you play enough games, the skill level of these players will approach the natural distribution, or average, just like when you flip the coins. What is the average, you ask? It's the average skill level of your rank. By this logic, the other players you play with will, over time, be constant in skill level. You will get games where your team sucks ass and you get rolled, and you'll get games where the opposite is true. I'm not saying just because you are better than the other players in your game you will win every game. Some games there will be someone trolling, making your team lose. But that person is just as likely to be on the other team, unless you are somehow influencing that event. I doubt you are.
So if the other 11 players are constant, the enemy team will have 6 of them and your team will have 5. Are you an average player for this skill level? Then you'll have equal probability of winning this game. On the other hand, are you better than this skill level? That means your team will, on average, have 5 average players and one good player, while the other team has 6 average players. You do the math on who wins most of the time.
The reasoning above is not 100% mathematically correct but it gets the point across.