Right before the flight (and upgrade list) goes to airport control, there's usually a final sweep taking all but 1 or 2 F seats based on the old upgrade list. So if an EXP gets on the list before the sweep, they might get it. If they get on after it's gone to airport control then there's only 1 or 2 seats left, and if another EXP or two shows up on the flight with a higher priority (for whatever reason) they can miss, even though platinums were upgraded just a few hours before.