Steam counts as "hours playing" all time that a Steam-launched app is open. That means that if you're AFK in a game, it counts towards Steam hours. If you pause it and have dinner, it counts towards Steam hours. If you just left it open on the title screen, it counts as Steam hours. If you have 3 games open, it counts as 3x Steam hours.

Considering that Steam launches itself when your computer starts and many people have multiple games, it's stupidly easy to run up super-high Steam hours counts. I've got over 1100 hours in Factorio, of which probably 1/3 were while I was sleeping, 1/3 while I was busy doing work or chores, and 1/6 were while I was checking Hacker News with Factorio idling in the background.

The top numbers are clearly bogus, Steam hours are easily manipulated and the top 5 exceed the length of time Steam has even been recording the data (started in 2009).