Mysterious ways of Revenue Management at hand.
You got an upgrade into an unsellable seat, @
McScruffy. It is conceivable that someone on your flight is so willing to get to wherever on time they are willing to pay a change fee and upgrade that they don't want to give the seat to you. And/or RM are just waiting until some random guy off the street won't buy the seat. I don't know the loads right now (or care to look them up), but the scenario I described is entirely within reason (and logic).