I know it's unlikely, but is it possible the buyer has opened a case for the wrong item? Perhaps they bought one before, and it fell through?

So, even although they are perfectly happy with the one they got from you, they may have had a bad experience with another one, and accidentally opened the request on the wrong listing?

It is unlikely, but you haven't said that you have been in touch with the buyer. Perhaps you could drop them a note, just to clarify...