Winning a lot to be in the top 5% MMR.
If you win like 25-50 games in a row without losing, you should almost definitely get there.
It's an MMR system just like HoN. Win% doesn't matter because you might win games not worth many points, or you might lose games that are worth a lot. If you go on winning streaks, you're likely to get much more valuable matches to win.
Originally Posted by theming
The OP asked a simple question and got a simple answer.