Welcome to WebmasterWorld Guest from 220.127.116.11
The reason is that URLs were picked from Google AdWords so they had the "gclid" variable applied.
I would think that Google would be smart enough to figure this.
Do you see this happening in your Webmaster Tools, if you're having any kind of query string attached (stuff after question mark)?
I haven't had such issues before, although I have been using AdWords for long and always had some kind of variables applied to the actual URL.