In February, Google Personalized Search got out of beta and was enabled by default in every Google account. To personalize your search results, Google uses more sources, the most important one being search history. You can pause or even delete the search history, but you may find it useful for future reference.
If you don't want personalized search results, Google recommends to log out, but this may not be the best option if you use other Google services at the same time (for example, you edit a document in Google Docs).
The "deus ex machina" in this story is the pws parameter that can be added to the address of a search results page to control the personalization.
This URL corresponds to a search for [Google blog] without personalization:
Try it when you're logged in and compare it to:
How to temporarily disable Google Personalized Search? Add &pws=0 in the address bar at the end of a Google search URL. Or drag this bookmarklet to your bookmark toolbar: