I think it is called Yield Management.

Where you checking online only? I just got off the phone to UA over here and booked the summer holiday. Interestingly the international availability jived with ual.com in expert mode (i.e. I saw O5 I5 and the agent said "oh, we have 5 seats for upgrade on that flight"). However, for some domestic flights, I saw zippo, but the agent had loads of available options on direct flights.

So, I would say give them a call and see (which is the ultimate bottom line with UA).