The gate allocation at T3 is done by a centralised stand planning section by BAA. BA for example puts in requests for use of certain gates for the longhaul flights and then wait to see what gets assigned. We don't always get our preferred gates, but we do get pier 5 (gates 23-42) that we prefer. Not many airlines like pier 7 (gates 13-22) and it could be that with many airlines having moved, AA has arranged for more of its flights to be in pier 5. IIRC eventually pier 7 is to be either renovated or rebuilt completely. It is a horrible area that feels very sterile and claustrophobic. I personally have only been down to pier 7 once and hope never to have to do it again. I know QF often gets gate 18 and many of their staff don't like it.

It is indeed easier for airlines if their flights are kept in similar areas, particularly those that do their own ground handling as it means not having to constantly move equipment around the T3 campus, which is large and cluttered. While many dislike the fact that all BA T3 shorthaul flights are coached (and I personally am not a fan), from an operational perspective it means that we get to keep all of our equipment in one area which has brought the benefits to the operation. Similarly, having our longhaul flights in the same general area means that we don't have to worry about moving to equipment to the other side of the terminal.