Perhaps it may seem arbitrary to line "in line pockets" but I disagree.
One thing about the Army is vast paperwork for anything to the point of deforestation. It can get pretty barren and meaningless but it is one of those things Soldiers have to put up with like boot camp (or officer's basic course).
It's as silly as push ups, sit-ups and the two mile run at times.
Look at what they did to the .30-06 to get to the 7.62 or the .222 to get to the .223--it all seems so arbitrary!
Buried underneath the Charles Dickens guillotine carts of paperwork there is a reason more than likely to end up justifying the difference.