OK, so I figured out what's happening.

In most markets, VAP30UPN would, in fact, require inventory in V along with the prime booking class (P, in this case). However, in EWR/LAX and EWR/SFO, UA has published this fare without the secondary inventory check: you can book VAP30UPN on any flight that has P available, assuming you have a 30-day advanced purchase.

Compare the booking code data for VAP30UPN, earlier in the thread, with the similar V-FN fare on AUS-SFO:

Note WHEN V AVAILABLE. OP's fare doesn't have that.

The key to the mystery is that when the through fare is very expensive, UA will check mixed class fares also, which necessitate using divorced inventory data. UA's married inventory for OP's flight is J9 C9 D9 Z1 P0 Y9 B9 M9 E0. However, there's P space on the SFO flight, when divorced from SFO-LAX, and UA finds the mixed-class price using the VAP30UPN fare, because the only booking class requirement is P and Y.

You can actually simulate this any time you want with a multiple-city search. If there's P on the EWR-SFO leg, you can connect it with any SFO-LAX within 4 hours in Y. (It also works for EWR-LAX-SFO).