Average Time on site Totals Issue

I've been looking at the different documentation on how average time on site is calculated however I'm confused by how the Average Time on site Totals is calculated (in screenshot 00:09:16)
From my understanding If I'm looking at a page name for my dimension, the average time should be calculated by a time stamp of each of the page name and then transformed to what we see (00:01:20,00:01:20,00:00:31..etc).
However, I'm confused how the Average Time on site total is calculated because if I do the calculation on my own it's not 00:09:16 but the average should be about 00:01:19. Why is it so skewed?
Field DescriptionWhat product are you using?What area of the product are you using?What are you trying to achieve?What’s the problem or error?What have you tried so far?Environment / VersionScreenshots / Videos / Logs / Code