Is the above pricing for upgrading roundtrip, or just one-way? You say they were both X fares outbound, but what about the return? How do you know
if the C+ inventory was the same for the return flights. Without screenshots, it's kind of hard to understand what you were seeing. We know that they must use some separate bucket inventory or pricing for post-purchase upgrades since people have been getting difference prices from what they see for upgrade pricing at time of purchase (which always seems to be just the difference between their selected coach fare and the cheapest C+/F fares).
I was only looking at upgrading one leg of the trip for both people (PVG-DTW) on the same flight and day. They have identical outbound itineraries (PVG-DTW-MSP) Including the outbound fare basis codes in the confirmation email.
I think what the previous poster said about one qualifying for a sale fare may have something to do with it, and somehow the return fare was controlling the outbound upgrade pricing. One is returning in December and the other in January. The one with the earlier return had the higher price quoted for the outbound upgrade.