Another data point. A coworker and I (both DM) had to rebook our trips home and it was done by a TA at the "same" time. On the second leg I am above him on the upgrade list (1 and 2) but on the first leg he got upgraded already and I am
#1 of 1 for 2 seats. How did he get upgraded for the first flight and I didn't when I am above him on the upgrade list for the next flight?