We spell that word aluminum, not aluminium. That's why we pronounce it that way. Why we spell it differently? No clue.
Also it's not just a billion. A trillion is a thousand billion on this side of the pond, and has been since well before any Americans were even close to being billionaires. We just use a smaller standard for counting, but that's also the standard French and Latin used, so I don't think it has anything to do with us.