I'd like to add to Andy's answer that with today's Google you cannot define what "natural position" actually means. Get on the phone with a friend who lives at a distance, and do the same search at the same time. As you compare results, you're going to find all kinds of variation for many searches.
Some factors that contribute include: geo-localization (ISP location), personalized SERPs (when logged in), different data centers, and sometimes just different browsers -- even in the same room!
So I encourage you to keep a focus on traffic from search and how well it converts. Do that more than focusing on rankings. Use rankings more as a diagnostic tool to understand and help your traffic improve.