I don't find the check-in staff really know why seats have been shifted. When I was last shifted, in October, it was from 10K to 11K on a 4-class 777. (Why was I in J on a horrid 4-class 777? - God only knows as I could have been in F for another 10k miles, but you live and learn).

At check-in they volunteered the information it was due to an "equipment change"; I didn't ask. When I got on board the plane was still a 4-class 777 and it still had a seat 10K. It was empty.

I asked the Purser politely, in the middle of the flight, if he had any idea why I'd been moved. He hadn't, and he believed seat 10K to be in perfect working order. Seat 10J was occupied. I didn't stare too hard but the occupant didn't look like he needed extra space. Maybe his guide dog was curled up quietly in 10K throughout the flight. What a nice thought.