The UK version seems to be exercise. I think it's just a cultural practice. For example, UK words that end in -our changed when brought to the US, and they now end in -or (e.g., colo[u]r, valo[u]r)

:)

J

Actually, the -or etc. spellings that arrived in America were current in Britain at the time - it was the British English that frenchified their spellings later on :)

Moog is right, albeit said in an uncalled-for way. Jayrio is wrong despite his cheerful response, however.

"Excercise" is a misspelling in _any_ part of the English-speaking sphere, especially in the US, where the developer, Matt Reagan, hails from. (Oregon)

According to the prescriptive OED, "-xc" spelling was only used in Middle English when spelling was still in a great state of flux, e.g.,

I rase fra ded til lyf, fra ydelnes til excercise in godis seruys. (1340)
He may putte hem from excercise and occupacion of the Kinges service. (1432)

Since the 1500s, the "-xc" has always been spelled simply as "-x".

Sorry, it's indeed a typo. No getting around it.

prescriptive → descriptive
(I don't mind OED to prescribe any rules, but rather just describe usage trends very comprehensively.)

See, everyone makes typos. It's important to own up to 'em. 　;)

mind → find

Goddarnit! Where's the Edit button on this thing‽