The cheaper A-fare on UA makes perfect sense.

AA has a P-fare (based on a K-fare KKX0C3B) for about $4,800 round-trip in the NYC-PVG market that books into F the whole way. UA could simply be matching with a limited number of seats or v.v.

Still, it's reasonable to see UA's Z-fare stable and not discounted further, to prevent re-fares on previously purchased tickets in Z.