I don't know. I suspect it has to do with the some dimension beyond the basic circumference of the head. But here again it might indeed be an administrative mark signifying the mold or maker.

Comes to helmets, ghastly things, needing one should be a clue that you are not wanted and desperately need to find friendlier neighbors. Mostly not worth the bother to stay if you don't have a huge uncompromising military regime forcing you to stay where you are not wanted. Sill a questionable item. Particularly the plastic sort.

Forced to have one you can give me a steel pot and liner. less effective as armor it is something that can justify its cost, weight, and bulk by being useful. Something you can dig with, boil up some coffee in, cook dinner, wash in, and sit on when the day is done. A real multi-purpose survival tool. Possibly the most useful thing the US military ever handed out to the common grunt.

Kevlar might be moderately effective at stopping a small, half-spent, bullet or fragment but its mostly useless for anything else.