Was there aircraft configuration change (bearing in mind it's possible that it had been changed and then changed back to what it was before), and the reason for seat shifting was recorded incorrectly?

Or, I suppose you may have been bumped by a Premier or BA Gold. I do not know if this actually happens, but being a non-BA OWE, if they do bump me for a Premier or BA Gold, I completely understand as their hierarchy is above me as far as I'm concerned, although of course I prefer not to be bumped.