Google has made some adjustments to how it uses referral URLs for contextual matching of AdSense ads. Google has started expanding the use of query words in referral URLs to a few hours. Basically what this means is that Google is using user's search history to determine what ads to show on sites using AdSense. If a user arrives at one AdSense site via a search results page, and then goes to another AdSense site within a few hours, they might see ads based on the referral data from the first one.