How is this possible?
This is pretty common - Google (and other PPC engines) always (in my experience) show a different number of clicks than the number of clicks shown by a client's stats program. There's a good few reasons for this - how cookies are handled, how long a session is, dynamic IP's etc.
As a rule of thumb, I usually let it slide as long as I'm reporting more clicks than adwords and the difference isn't dramatic. I have yet to see Google (or Overture, Espotting, Mirago) report the same amount of clicks as a stats program.
If you're just looking at people coming from Google, remember that AOL/AJ also sends you AdWords traffic.
Do you have content syndication turned on? If so, that traffic will be coming from websites - not search engines.
There are other browsers that let you tailor all sorts of stuff that gets put into your logs. I get a kick out of the folks that visit my site who report their computer and OS as things like "TRS-80" and "CP/M" :).