If you look at the fare filings on EF, you will see AA now has a number "-UP" fare filings in PE from certain markets likes BWI, LAX, MCI, etc. These are not true P/W fares, but rather coach basis fares with a /PAUP ticket designator and a P booking class. There is a common component to these fares in that they all seem to require Sun-Thu departures while the true P/W fares do not. While it's not quite clear what inventory the system is looking at to determine when the domestic segment is bookable (they all seem to book into Y booking class), it looks like it may use availability in the underlying fare basis bucket for these -UP fares thus making it easier to find availability with Sun-Thu departures.