Originally Posted by
sethb
If it's not at the gate, they have a third option: Take nobody yet.
If at the gate, they ask the next group if they want to split.
Which is probably what happened here, and hence why one RU was still available when the solo obtained GM status. If his ticket was bought first and then he got HM, presumably he may be (correctly) above the two-person PNR on the upgrade list at that point. Since the next solo on the list
wasn't upgraded at the window, then the upgrade was available once the solo got their new placement on the UG list.