It's ranked on a percentage basis for divisions based on team performance (even if you play solo). Masters is top 2%, Platinum is the next 18%, Gold, Silver, Bronze and Iron split up the other 80%. What this means is you can sit at the top of division for a while if your performances aren't on a par with players in the higher divisions, it isn't just a matter of being automatically promoted once you top a division or earning a certain number of wins to move up. The only real way to move up is to consistently beat higher ranked players. It took me a 12 win streak to move from Platinum to Masters to give you some idea. Get a good team together and put in a good run of wins and you'll move up no problem.