[ubernostrum]

Or somehow Britain and its overseas colonies didn't have any standardized spelling for words for a long time, and so spellings fell in and out of favor and whatever was in vogue in each place at the time the earliest widely-accepted dictionaries were compiled became the "right" spelling for that location.

(speaking of which, Commonwealth English has a stronger tendency to hypercorrect and create false spellings which actually differ from the imagined root; "foetus" is the classic example, since the Latin root was actually just "fetus")