Here is a little bit of a article I was reading about zombies:

People play Zombies and have no possibility of ever really wining; length of survival is the only real victory. This shows great investment on the part of Zombies players. This was our motivation and direction on how we should honor that commitment. Rank in Zombies is a live reflection of the player actual skill in Zombies. It will rise and fall as the players abilities increase or decrease in game. This allows the community to see just what a particular player might bring to the team. This also allows us to match make based on skill and create better teams, which means better more consistent games. And with everything Zombies, there is also a bit of mystery embedded in the ranking system so fans might discover something extra as well.

This leads us to belive it is about EVERYTHING and your rank could fluctuate.

Here is the link to the interview: [ Register or Signin to view external links. ]