What an American Airlines Reservations Specialist told me was that Orbitz.com buys seats in a particular fare class such as N or L (the deeply discounted economy fares) so even if you select the Exact Same Flights (time of day) coming and going on aa.com or by calling AA versus using Orbitz.com, you can, occasionally, come up with only the higher fare classes such as V (still discounted, but not as deeply).

What I always wonder is if Orbitz basically takes a loss on the seats they don't sell or if they're allowed to give back to the airline whatever they don't sell, since Orbitz is buying seats en mass.