My belief is that you need to keep progressing.
So say you got to level 30 in NTZombies, with 2,000 kills, 150 head shots, 3 doors, 15 revives, etc.
In order to "rank up", you will need to do better then that on your next round of NTZombies in order to keep "ranking up".
So you will need to make it past level 30, kill more then 2,000 Zombs, get more then 150 headshots, etc., and that will keep you progressing.
I got this theory because I am sitting at Skull with 3 slashes. I played yesterday for about 3 hours, but didn't make it past any of my higher rounds, and that obviously leads me to not getting as many kills, headshots, revives, etc. and thus after 3 hours of playing, i was still sitting at skull with 3 slashes.
This is just my theory. I wish there was something more concrete the DEVS would release, but I think they want it this way. More mystery and such.