I've always had a lot of trouble with the time tracking/last played fields, so I just don't rely on them at all.
An example I just had was a game that I had set as "In Progress" and then moved to one of the "Completed" options. It went from seeming to track my playtime fairly accurately actually while In Progress, around 18 total hours for a game, but when I changed it to Completed all the game time went down to zero and it's acting like I've never even played the game.