Hey guys.
After a game I just played I'm kinda led to believe that the zombie rankings are based on level. Not Skill.
Nearly every game I play I am always top of the scoreboard when its either 1, 2, 3 or 4 players.
I always have most kills, Most Revives and Most Head shots. But every friend I play with manages to have a higher rank than me. I'm higher on the leader boards than they are and I'm a lot more consistent with my games than they are. (They get to round 15 nearly every game. I get to round 20-25 every game.) But after this game i had just played (Like 5 minutes ago) I finally leveled up. I got to round 31 with a friend and then my game froze and had to turn my Xbox off.
After i came back on I had leveled up from the skull. To the skull with blue eyes.

So I am beginning to believe that the Zombie Ranks are not based on skill, but on what your highest rounds are.
I am going to try get to round 40, 50, 60 ect. and Ill report back with whether I rank up or not.
If I do then I will be almost 100% sure that it is based on rounds.

Hopefully this kinda gives people a little help.

Thanks.

EDIT:
I have started playing a lot more 'Grief Mode' and winning a few of those seemed to get my rank from the normal 'Skull' to the 'Skull with Blue Eyes' rank.

Also then, I began to just play 'Survival' instead of 'TranZit' and after playing one game and making it to round 31 I managed to get the 'Skull with Blue Eyes and 1 Knife behind it' rank.

So I would say it could possibly be based on winning 'Grief Mode' matches and also making it to higher rounds on 'Survival' instead of 'TranZit' game mode.

EDIT 2:
The ranking system is most likely based on your past 5 games.
Which would explain the 1-5 tally marks you get. You could possibly earn them by having a 'good game' which would mean if your past 3 out of 5 games were good(I'm going to say above 20-25) then you will get a tally mark. Which would mean when you have 3 tally marks that means you had 3 'good games' out of your past 5 'good games'
I'm going to say you have to get over 20-25 to get a tally mark. So once you get 5 'good games' you will get blue eyes, and if you are more consistent with these then you will rank up.
THIS IS NOT CONFIRMED. IT IS WHAT I AM LED TO BELIEVE FROM ALL THE PAST GAMES I HAVE PLAYED.
Also if you are inactive on zombies for over a day or two you will be de-ranked. I was away for around almost 2 days and I was pushed back to the skull.


WAYS TO GET DERANKED:
1. By not playing for a consecutive no. of days. Eg. 2-3 days.
2. Leaving when you are in mid-game.
3. Getting downed ALOT of times in a single game(20+)
4. Playing bad overall for a consecutive no. of games.