But *why* does the spelling throw you?

'I actually use a Canadian beta for dS to make sure Fraser doesn't do/say anything too USA'
But do you also change your spelling to Canadian? - it's spelling I'm asking specifically about, not idioms. I understand that Americans shouldn't use British idioms, but spelling...?

I read a lot of fic and I've never been thrown out of a story by the spelling (unless it's really bad. (o:). It seems to be more of a problem for American readers than those of other nationalities.