Another scenario is that the aircraft was reported as having a number of defective seats, and so the passengers were moved out of them. Then they were repaired or sorted out in the run-up to departure and thus re-entered service. In so doing, people were not necessarily re-assigned to the seats that they started with.

There is a Flub version of this, where people are moved into Flub since there is no option (either due to defective seats or overbooking), then at check-in close it turns out there are some no-shows or people stuck on unviable connections, so people are then moved back to CW since there are now enough seats - but not the seats they chose long ago. So a GGL or Prem may end up getting a seat they would never have chosen, while the seat they did choose has been given to someone without status upgraded from an over-sold WTP cabin.

The way this works is partly offline, so in the latter case the GGL may or may not have been told about the Flub, but would definitely be told about their new and inferior seat.