I had the same problem last month on a flight from VIE-AMS-SEA-XXX. Checked in at Vienna was #1 for 8 seats. Time I got the gate at AMS I was #1 for 0 seats. Was told they sold 8 seats in the last two hours.
Heck things where so messed up I wasn't even on the upgrade list for the domestic flight out of SEA. Before you ask it was a different flight number than the international flight. Delta told me it was an IT glitch.