Think of me (a Masters DPS) in a Silver/gold game… My MMR is drastically higher than that of the majority of other Silver/gold players. So in order to effectively match me in a game, the MM has to find another player (or group of players) that will compensate for my performance, and put them on the enemy team.
The problem is, if I’m smurfing, there really aren’t that many, equally skilled individuals or groups of individuals, who can match my level of performance output. So the game creates the match with my team having a varied % chance of winning the game.
According to everything I’ve seen, the Match Maker is allowed to create matches ranging from one team having a 40% chance and the enemy having a 60% chance, where obviously the preference is a 50% chance on both sides.
Follow this link and follow the link that WyomingMyst references.