Maybe you left the game(s) running longer than you meant to?

Well, I know that my playtimes are inflated, but for different reasons.

First, there is the game itself. Games do not report any kind of activity to Steam, so when I Alt-Tab out of the game, I'm still playing as far as Steam concerned. I might even forget that I still have that game sitting in the background...

Second, there is Steam itself. According to Steam data, I have played a lot of games for a couple of hours. In reality, that was just to drop the cards.

Last, there are external reasons - like the classic "play x for y hours".

You feel the hours don't match up or you see that you've played X amount of hours and then see something else as a result in Steam?

The Finals, for instance, has this metric but only while you are mid-match. But Steam will show the total time spent mid-match and in the menus.

Time is tracked by the game exe running so that would count such things as leaving it running while you go to take a shower, go to the kitchen to make a cup of coffee, receiving a phone call etc.