Sometimes a seat is notified as faulty and the person booked into it is moved. It is then repaired and a new person slotted in rather than move the original person back. This by it's nature are would fit into a last minute change from you and to then someone else.

I have had a faulty 2k in 747's on several occasions and each time was given on board CSD/CSM Avios credits. Last year it was quite a significant number as it was 2 flights out of 3 747 flights. On one of them last year we were given several different "reasons" and it was annoying because the person ending up in "my seat" was a BA Captain who received quite lavish love from cabin crew.