Google is helping its users understand what's behind customized, or personalized, results (as opposed to Google Custom Search). They're rolling out a new feature that explains why results may be tailored to a particular user.
The feature will be rolled out over the next few days. When it does, you'll notice a new link for "More Details" in the top right corner. Clicking on the link will take you to an explanation of customized results. The explanation says that location (IP address), recent searches and web history to serve up the personalization.
Google noted that this feature does not change how they compile results.