The data used to calculate it may include sessions that you break with redirects or links to pages that don`t carry the Google Analytics code. These would be "hidden doors" that distort analytics data. Many Websites contain them.
It could also be that you`re not properly filtering out rogue bot activity. Traffic faking bots could be recording false hits and they won`t last very long, so they dilute the value of any real session data.
And it could be that a substantial number of your visitors quickly realize your blog isn`t what they are looking for so they leave or close the browser session immediately.
Or it could be that many of your visitors are connecting over poor connections or that your site`s server goes offline more often than you`re aware.
You should look for other signals in your data to help you determine how satisfied people are with your content.
Michael Martinez: If I suspected something was wrong I would look at the audience demographics and the source/medium reports and see if I could determine a consistent source or pattern for the short duration visits. You should also look at page-level data. Every Website…