The back of the helmet gave me all sorts of trouble. When I originally made the helmet, I don't think I had the right proportions, so there wasn't enough room for my head to fit. Therefore I just tried to make it work with what I had. I tried to keep the right detail, it just looks kind of warped. I also didn't want the helmet to be too large, so I don't really want to go back and fix it, and risk making a bobble head Master Chief.
The bottom line was that I could have the helmet be a perfect replication, but too big for the suit, or the right size and not perfect on the back detail. The back of the head doesn't get much attention, so I figured it would be fine the way it is.