If I were on your flight, all you would need to do is tell me that the FA's had put you in the seat your were in. As long as I knew that you didn't reseat yourself (i.e. seat poaching), I wouldn't debate the reasons with you.

After that, I could take it up with the FA's if I felt that something wasn't right and they could explain the situation and/or accommodate me as appropriate. Most likely, I'd want to talk to them anyways to find out where they wanted me to sit.