Question about GMail referers

When I get referers from GMail messages on my new blog, they often contain a query string parameter labeled 'cat' with a cleartext, meaningful value in it. I've often been able to determine, from the 'cat' value, exactly who is talking about my site in email, and in one case, exactly what they thought of what we're doing! (Fortunately, the news was good.) In other cases, the information has been more general, but still meaningful (for instance, the name of a mailing list to which I sent a launch announcement).

I don't use GMail, so I'm not sure exactly what 'cat' is. Labels? Search terms? Any ideas from the GMail crowd? I also don't understand, at all, why I would be getting this information. I should not be seeing any information people are using to organize or search for their mail. (Yahoo Mail and Hotmail both have meaningless, to me, URLs.) Anyone know why this would happen? The Google GMail privacy faq says:

Google also takes several steps to guard the confidentiality of users' information by offering a number of industry-leading protections. Among other things, Gmail users benefit from: [...] Minimized "referrer" header information. When you click on links in messages, the web browser that loads contains a referrer header. When you click on links in Gmail, Google takes steps to eliminate this referrer header, preventing others from knowing that you clicked on a link from an email.

Hmmm....

Update: I can't believe I missed the opportunity to title this post, "The cat's out of the bag."