On the ranked matches on Xbox live, how is your score determined? I am fairly sure it is a "true skill" ranking, but it doesn't seem to be dependant on number won or any other obivious stat.
Basically the system tries to predict if you will win or loss based on the skill level of the players.
An very simple example is:
Sometimes you need to win thousand matches to rank up, and sometimes loss just a single one can put you down in the rank.