2 hours seems to be the final dump for the VX metal flights.
AS metal flights seem to release a lot more U ahead of time.
I've been on off-timed lightly loaded VX flights with the same scenario (make us sweat it out until the 2 hour mark, dump the cabin for upgrades, and fly with empties or non-revs up front since there are no or few other Elites).
In the end it looks like RM incompetence (or best case, unfamiliarity with the loads at given times). At the same time, as a SFO-based customer, if I'm going to pay for F, I'm going to pay for VX metal. So I can see the rationale in holding so much VX metal back.