nullPointer wrote: AFAIK, it doesn't take into account which games you may have already played and beaten. So the playtime you have remaining in your Steam library is potentially much different from the number being generated on the site.
That's a good point. It would have no way to know you'd beaten something unless perhaps you had every single achievement for a game, then it might be able to assume that somewhat safely. Of course it has to be using that 'time to beat' estimate page and then there will be games that you can't necessarily beat like MMO types and some of the rogue-lites. Still an interesting and possibly frightening comparison.