lucy24 - 5:23 am on Mar 16, 2013 (gmt 0)
By odd coincidence I have just noticed the identical phenomenon in a context that has nothing whatsoever to do with google.
In my case: The piwik package lives on my main site, but also tracks a second site. Browser #1 is set to "accept cookies only from sites I visit". Browser #2 is set to "block cookies from third parties and advertisers". (Browser #3 is Firefox, but so far I haven't found* the cookie prefs setting.) And yet if I go to Site B in either browser and then check cookies, I will find cookies with both sites' names on them.
It made me wonder if possibly "third-party cookie" means something different than what we think it means.
I don't see how it can have anything to do with g###, though. Cookies are at the mercy of your individual browser.
* After posting, I investigated further via the FF Help site. Answer: Selecting the "Use custom settings" History option will reveal a bunch of new clickboxes. Honestly now Firefox, do you think any human being anywhere would figure this out on their own? Grrr.