Not all referrals from Google.com domains come through organic search or AdWords ad listings. Referrals may come from a variety of sources, including Google Groups posts, or static pages on related Google sites. Such sessions are tagged as [referral] instead of [organic] or [cpc].
Note that under some circumstances, the referer field may be blank. For example, if a website is accessed from a HTTP Secure (HTTPS) connection and a link points to anywhere except another secure location, then the referer field is not sent. (Source: Wikipedia)