It's all in the context of the usage.... when you are looking at a monthly breakdown, the time spent is showing the average time spend across the entire site per visitor (so every page for every visit within that month is taken into consideration). The total at the top will average out the time spent by each visitor for all pages.
When you break it down by page or URL, the average time spend by every visitor for each individual page is being looked at.
So let's say you take a generic page, like your home page... the visitor may have visited that page 3 times, the average time would show the average time spend on the home page within your selected time frame.
But maybe there's another page that is less popular, like maybe a specific content page, that they only view once.. the time there will be the time spent.... The total at the top will take the average time for each page individually per user, then averaged across the pages....
The "name" Average Time on Site is a bit misleading, since it also represents average time on pages or other breakdowns depending on the usage.