Weird Google Site Search Results

My collegue Harald made me aware of a problem at Google.com. When searching for a specific domain (site:xyz.com) Google also returns a few results from what looks like random domains. Very often these include splogs (spam weblogs) and other MFA (Made For Adsense) websites. I found that it only occurs if you're not logged into your Google account.
What are those guys at Google doing?!