Maybe all planes don't need 15 skins, and some planes need even more.   For example, the Wildcat has been around for a long time and doesn't even have half it's skin limit.   And are there even 15 different skins for the F4U-1C?   But on the other hand a very popular plane like the P-51 with a lot of very colorful and different skins possible, perhaps deserves more slots?