Here's a scenario I can make up.

Imagine you get the last Y seat and you're checked into seat 45F. Meanwhile some other passenger booked into F has checked in and got seat 4E. 1A remains free because 1A is not going to show but BA doesn't know that, they're just lost in the terminal somewhere.

Sometime very near checkin and boarding the other passenger gets shifted to 1A leaving 4E free. She doesn't know this and sits in 4E. You arrive to sit in 4E and you stare at each other and you shuffle off back towards row 45. However, a standby passenger boarded after you and went to 45F so that seat now has someone in it. Ontime departure is critical... and there's a spare seat in J. Please sit here!

After takeoff the CSM reviews the situation (and maybe has updated data on the iPad), discovers 4E is free, moves the other 4E occupant to 1A and sends for you to be brought to 4E.