We call it playing chess unpredictably as opposed to playing variants of chess because they are not different games. These are updated rules that improve chess, just as en passant. promotion and castling improved chess, but did not create different games. Within the rules both players could agree to play with the well known RNBQKBNR arrangement of their non-pawn pieces on their back ranks. I did not know about placement chess when I started this website. After learning about it, I have concluded that the rules are almost the same, but not as good, due to playing chess unpredictably having better castling rules and options for increased symmetry.