Hmm, it definitely seems swap related. Even more strangely, UA29 tonight (a 2-class 763) is showing booked 20/30 but is J2. Even if they swapped to a 3-class 763, that should still be 26 seats. {it was pointed out this is just over by 8 in Y}

Unless there is some married connecting market I'm forgetting? But blocking eight seats from inventory 4 hours from departure seems insane.