The USMC is mandated to operate a few squadrons off of CVNs, and the Navy is not going to let the Bravos melt their decks (CVNs are not in line to get the new thermal coatings, reinforcements, and equipment relocations needed to operate it) or otherwise interfere with cycles. The agreement with the Navy to acquire 80 Charlies was made prior to the decision to upgrade and SLEP USMC Hornets. Between that decision and the earlier agreement to buy 80 Charlies, the Marines shaved off 13 - now they want only 67 -Cs - and upped the Bravo order by 13.