Are you verifying that the traffic is coming from the regular search results and not something like the language translate pages? I know it sounds weird but for whatever reason I often notice massive hits from the translate pages. Far beyond what would be normal with real people translating pages. I suspect it might be a type of bot/scraper technique? If you get hit every once in a while by some crap like this it might explain it. Certain stats programs might count this as regular SE traffic.
Example:
http://translate.google.com/translat...fy.com&act=url
Notice the referrer will be from *.google.com or such.