then just "perceieved skill" of player will dictate that, ie som1 who is perceieved as 1400 but is 1600 will give him wrose team balance overal than if it was opposite. so it may just be that weirdjokes, darkcity, vinme are seen as less skilled than they are, so when game is balanced they are given advanatageous teams.
also depends im sure on players database, ie whoever they play with more, and whoever playes better when playing with them.