Seems kind of arbitrary to me. Is there any particular reasoning here or is it just a quirk of the rules?

I guess it’s mainly because ASM’s more proper name would be “Any% No Mission Skips” and so we treat it as such, whereas 100% is just getting 100% completion as fast as possible

i'm fairly sure you can skip missions in ASM as long as you go back and complete them like in 100% but what would be the point it would just be slow.

^ Skipping missions is outright banned in ASM, but it's pointless regardless since there are no known cases where going back to do them in a faster car would save time.

100% requires completing the game as fast as possible, so skipping missions to complete it with a faster car later is allowed, as long as you do complete the game at 100%

All story missions is designed to complete the missions in sequential order, and there are no cases where coming back with a faster car is worth it anyway.