Originally Posted by Td0t
...I realize that flights fill up fast on weekends, but there are still dozens of seats left on this particular flight. I assume that BA has access to a limited number of seats available per AA flight. BA's quota must have been reached. ...
Not true, BA has access to the same inventory at AA. The yield management software at AA is quite famous, it is very good at predicting demand, including last minute purchases. This allows AA to oversell some routes, and of more interest to you, to leave seats free until quite late (confident in those last-minute sales materializing). This also results in considerable differences in award availability by date. You can check up to the day of travel, and AA will convert empty seats into availability for awards (at least it does for EXP).