It's not just theoretical -- IIUC, if Charles, William and George get hit by the same bus, Charlotte would become Queen of the UK while Louis would become King of Canada.

I'm sure that in that scenario Canada would quickly update their primogeniture laws to give Charlotte primacy over her younger brother, but at the moment the primogeniture laws of the two countries are different.

No, all the Commonwealth countries changed their succession laws at the same time precisely to avoid this scenario.