Visitor Time on Page calculation by Google Analytics doesn't make sense
blog.simpleanalytics.comI think in general all website metrics are just an approximation. Even "real" sign-ups are sometimes incorrect as some users might sign-up multiple times, bots might sign-up. Maybe conversions/purchases and money earned are the only hard metric, but that also has to take into account refunds, chargebacks (fake card used).
That being said, most of the time those approximations are good enough to suggest actionable steps.
Regarding this specific post, the user might have his browser tab open but go eat lunch and then come back, in which case the "time on page" is still not showing the time the user actually spent on your website. A more accurate metric would be to check for user activity, but then again, the user might be inactive for 1 hour yet somehow actively reading the page content.
Neither does the sum of returning and new users. They at least point out that the definition of "new" being "didn't let us do a cookie" (plus other stuff) explains some of it.
I find the whole thing of metrics measured this way, as pretty bogus. I trust their long URL clickthrough tracking. I don't trust much about breadcrumbs and cookies otherwise