instand1 - 6:40 pm on Jul 4, 2013 (gmt 0)
Google serves different positions to different persons. If a person never clicks on a specific ad, then this ad is shown less frequently. It depends on the profile that google has of the person. If you are logged in any google account, you get different results. (in SERPS and in the positions of the ads.)
Whether the Ad Preview shows the average position, if you use it often enough? Maybe.