Well, this is argument covering English versus new world English (which I prefer to call American). The spelling checkers do make a difference between English, US English, Australian English, South African English, etc. for good reasons. No reasons to argue about that. Those languages are different.
There are other language 'families' with the same deviations, think about Dutch, Flamish and South African. Also Spanish/Mexican....
Rob.
There are more examples, both in these two languages as in other languages