The account named "Administrator" that Windows created for you always had the chess piece.

There's no way my memory of this is reliable anymore, but I also remember my administrator account being chess pieces (and my user account being an orange fish).

It makes sense. The chess piece is the first profile picture in the list.

I remember it like that too. Or is it Mandela effect?

It might be that the randomization code was bypassed in some cases - like creating an account in safe boot mode or similar.