Check out this video about the Halo 4 progression system if you want to know what was released so far about the Halo ranking system.

Is it similiar to Halo 3? To Reach?

Is it going to be awesome and make you want to play even more?

http://youtu.be/JBRGa7bAdrw

Try to check the xbox 360 forums on xbox.com I remember someone had this problem and they had to clear out there cashe on the the 360 or something but I can't remember exactly how. A lot of people have this problem. It might ne that it didn't just register yet of you just got it, but I got that achievement when I was supposed to playing only execution matches.