Thanks to Baeck, formeraa and audio-nut for an excellent discussion of airline pricing decisions. It is a fascinating topic.

My guess is that AA probably has good reason (since only AA has all the historical data) to suspect that the particular flight desired by the OP tends to fill up at the last minute with walk-up fares, despite appearing only about half full with only 4 days to go.

Of course, plenty of planes depart from the gate at 50% load factors; they tend to balance out the packed planes, leading to AA's 75% (roughly) load factor. Even if AA were the best in the industry at managing its inventory to maximize revenue (I tend to think it IS the best at it), there will be some flights out of more than 2,400 daily mainline flights where AA boots the ball. This flight might just be one example of a less-than-stellar performance.