I have long considered saver First Class awards to be the best value for me, and it's nice to see that AA is willing to give away some F seats on the flight detailed by the OP.

I agree with the possible reasons advanced by the OP for the discrepancy; additionally, it's obvious that AA forecasts that J seats will sell more briskly than will F seats, and thus is willing to give away F seats while holding back U inventory for now.