Simsi - 5:53 pm on Jan 8, 2012 (gmt 0)
I wouldn't be surprised if the vertical you are in makes a difference. Looking at the "average" for that vertical and seeing how you compare carries some logic to it IMO. A 5 second bounce might turn out to be better than 75% of the sites in that sector for that term...trouble is you'll never know.
For example, in a lot of verticals, a quick scan of the pictures are enough to make a decision and that can be almost instant. Doesn't mean the quality of information is poor though.
Additionally, G might factor in information on the user and their general trends or even whether the search term included your brand which has to be a good sign.