Perhaps F was oversold. The person who wasn't on the plane had a revenue F ticket but no seat assignment. So they had more people assigned to F than seats, and they had to downgrade someone.

I've seen this happen over at United, where they downgrade upgraders (prioritized by status) before awards before revenue. And when they downgrade someone they issue a 'downgrade kit': they refund the upgrade instrument used for that flight (UA doesn't have "unlimited" domestic upgrades) and they provide a confirmed upgrade cert for a future flight.

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View from the Wing: A blog about Free Miles and Free Markets