My experience last week:

I called AA to book a DCA-ABQ First Class on AA for two weeks from now. Sorry, nothing available in either coach or first (that Sunday night return flight gets you every time). So, I hung up.

Then I called US Airways and tried to book the same AA seat using the exact same flights mentioned to the AA rep not 120 seconds earlier... "Sure, no problem, would you like coach or First Class???"

Don't ask me for an explanaition... I don't get it. The only possibility I can come up with is that I'm an elite with US Airways and not on AA.