Checked a few seat maps, on long haul flights, 772 and high J 763 will shows 1A missing, both in UA app GDS, while 789 and 77W won't. This explain why OP ride in 1A between EWR and NRT.
After more digging, I found UA16 switched to 772 from 763 starting Oct 30. My theory is when OP booked this flight, it was scheduled to operated by 763, so OP can select 1A. At some point equipment changed to 772, the system automatically assigned another seat for OP since there is no 1A on 772.