It’s up to the game developer to enable playtime tracking on Xbox, some do not support the stat so there won’t be any hours logged. The game itself might have a separate internal playtime tracker but the site will be unable to see that.
The hours played should be matching what is shown on your Xbox profile. So regarding incorrect hours - it's likely the same is shown there. Since we just read in the data from Xbox there isn't really anything we can do to improve the accuracy. I've found the Xbox tracking to be mostly accurate though, but it's possible certain games have issues yes.