> But can Google differentiate?
Yes, they can. By measuring click-throughs, bounce rates, and repeat click-throughs in a search session, for example. Doing so, they can quickly get a fairly good metric on how happy a searcher is with the link Google provided to your site in their search results. See all those www.google.com/url?cd=1&q=fuzzy%20blue%20widgets referrers? That's what they're all about.
Oh, and it's only "cd=1" if you're lucky... :)
Sometimes the issue is content quality. Sometimes it's the site's technical quality -- duplicate-content and infinite URL-spaces as hinted-at here. Sometimes it's many factors. As Tedster said above, the important thing is to *know* what that message might mean in the context of your site and its unique URL-space.