When doing SEO optimization, be careful of the personalized feature Google offers:

Google on Friday moved to combine two of its separate personalization features in order to make searching a bit more friendly for its users. When signed into the site, Google will automatically custom tailor search results based on search history and preferences selected for the personalized homepage.

“Keep in mind that personalization is subtle—at first you may not notice any difference,” says Google engineer Sep Kamvar. Those who don’t want Google to track what they’re doing can simply sign out of the site and personalization features will be disabled. “After all, the goal is to give you what you want when you want it,” added Kamvar. Source

You may rank well on your personalized search, but not well in the search results others are seeing. Make sure you turn off personalized search if you’re evaluating your SERPs position!