While doing research before I bought my truck I found thread after thread about this topic. If I remember right the consensus was that because of new federal emission standards that went into effect in 2008 the 6.4 gets worse mileage. From what I understand the 6.4 was designed and tested and was ready to deploy and it was kind of an after thought to get them up to the new emission par. They threw on the new equipment and shipped them out and found that maybe the new stuff wasn't optimized for the engine. There is obviously more to it than this, but this is the readers digest version to my understanding.
If you go over to the 6.4 section the first advice you would see is that you need to delete the dpf and the egr, and that if you do this you mileage will jump to high teens or low 20s.
I didn't want to deal with that so I bought an '07 and still get 13ish.