A video posted by McDonalds Canada reveals how they stage the burgers for photographing them. They shift each layer backwards (bun, meat, etc) so that the ingredients of the layer are more visible when photographed. The top bun ends up being a few inches backward compared to the bottom bun.
I used to work in food photography (in an admin role).
Sprayed on glycerine for condensation on cool things. cigarette smoke for steam.
It was super nasty, but the photos looked good.
However, the buns on the Japanese menu are much more obviously askew than other countries menus, which is the interesting point I imagine the OP was getting at.
Compare for example with the UK images which are much more symmetrical:
https://www.mcdonalds.com/gb/en-gb/menu/burgers.html