Well i think it's something like this:
Lets say you're silver league. You've been playing for quite some time and you're getting better. You start getting matched up with gold league players and you win all. Ok. But bnet wants to put you in a league that matches your current skill level. It could put you in gold, but you might be better than that. So it starts matching you up with plat and diamond players, until it finds a class of players that are around your skill level.
If you keep beating all the diamond players bnet throws at you, I guess it'll try to put you in a higher league, which doesn't exist so you don't promote. Or something like that. Search tl.net if you want more info on it, I think there's a thread there that discuss the promotion system quite in depth.
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