I've seen this sort of thing before, in both directions. I believe it stems from the fact that Steam tracks playtime by launching the app, and then paying attention to how long that app stays open.
In the case of some games (Thief 3 comes to mind), the app Steam launches immediately launches another app and then quits. So, your played time will look like less than a minute, even if you play for hours.
In other games, the app is actually a launcher that stays resident - in Anno's case, it launches an Ubisoft launcher/UPlay client, from which you can actually launche the game. If you don't quit out of this launcher when you're done playing, Steam will continue to count the game as being played. Also, you won't be able to launch the game again from Steam, as it will only allow a single instance of a game at a time.