In an attempt to provide more accuracy on where your site ranks for search queries, Google is changing the way it calculates average position in the Google Webmaster Tools Top Search Queries and Google Analytics Search Engine Optimization reports.
Heading forward, Google will average “only the top position that a URL from your site appeared in,” whereas previously Google reported the average ranking position of pages on your site that appeared in Google’s search results for a particular query. Google offered this example:
“Let’s say Nick searched for [bacon] and URLs from your site appeared in positions 3, 6, and 12. Jane also searched for [bacon] and URLs from your site appeared in positions 5 and 9. Previously, we would have averaged all these positions together and shown an Average Position of 7. Going forward, we’ll only average the highest position your site appeared in for each search (3 for Nick’s search and 5 for Jane’s search), for an Average Position of 4.”
Google emphasized that historical data won’t change.