If you view the actual per visitor times, you will see that they are much higher than 14 seconds. You have to look at what session duration is. If I come to your first page (1 session) stay on it for 14 seconds, and then go to another page (2nd session) and spend 14 seconds there looking at the content, the average is 14 seconds. However, that person stayed on your site for much longer than that. Those that came to your front page, read it for 2 minutes then left without going to another page count as a 1-second session since there is no data to compare between 2 points. So this session brings down the average. Again overall, those people who went beyond the front page stayed much longer than the average.