Canceled or Cancelled?

This one makes absolutely no sense to me.  And, it’s tripping me up.

I always thought “cancelled” was spelled with two “l”s.  That’s always the way I’d seen it spelled.  At least until now, that is.  These days I’ve seen it spelled more often “canceled”.  I don’t know why this is, but it’s gotten to the point where when I see “cancelled”, it looks like it’s spelled wrong.  I’d love an explanation, but I know it’s probably nothing more than personal preference.  But maybe this does have an explanation:  when did “canceled” become the norm?