What tool are you using where you found "average time spent on page"?
To be honest, the data is probably coming from "average time spent on site" (and just how the tool is pulling it, they gave it a custom name).. as this metric really isn't just "site", it depends on how you breakdown the data...
Using "average time spent on site" against dimensions like "Day" or "Month" will be the average time for entire visits, but if you use a breakdown like Page, then the calculation will be the average time spent on each page.
All "time" metrics in Adobe are calculated based on the time stamps recorded on each page view. So let's say the user does the following:
- Page A (timestamp 10:05am)
- Page B (timestamp 10:06am)
- Page C (timestamp 10:08am)
- Page D (timestamp 10:11am)
- Time spent on Page A is the timestamp of Page B minus Page A (so 10:06-10:05 = 1 min)
- Time spent on Page B is the timestamp of Page C minus Page B (so 10:08-10:06 = 2 min)
- Time spent on Page C is the timestamp of Page D minus Page C (so 10:11-10:08 = 3 min)
- Time spent on Page D is unknown, since there is no "next timestamp" to use in the calculation
- Time spent on the Site is the timestamp of Page D minus Page A (so 10:11-10:05 = 6 min)