I went into a lot more detail here, but my assumption is that players are distributed between teams in a 12 person lobby based on their personal performance relative to their rank.
This would mean anyone who belongs several ranks higher will have high enough impact regularly enough that nobody in the lobby can balance them. Someone who’s at the high end of the lobby, but not high enough to hard carry, is going to continually get poor teammates to try to make an even match, which will cause stagnation.
I approached it from the standpoint of a developer, and explained both how and why it could exist. If you’re not opposed to entertaining opposing viewpoints, please give it a read.