Why are Steam's "hours played" numbers wrong for so many games?

For many games on my game list, the number of hours I am "on record" for playing are much less than I've actually played.

My numbers for the game Altitude (bought in 2009, did most of my playing in the month or two after purchase) are accurate, while my numbers for And Yet It Moves (bought in 2009-2010) are about 10% of what I actually spent in the game.

What determines whether or not Steam keeps tracking my play-time accurately? Are the actual for-real gameplay hours stored anywhere I can get at them?

Is this behavior intentional on Valve's part, something to improve performance? Or have they been struck by accidental data loss on multiple occasions?

Edit: I always launch my games from within Steam, and I never go into offline mode (yay for living with always-on broadband).

For all the example games I gave (and others I didn't), I have logged in to Steam in the past and had it show me accurate playtime numbers. But then, for some reason, those numbers disappear at some point.

Edit: Here is a screenshot from my games list

• Altitude came out on December 4, 2009. I believe the hours played number is correct.
• And Yet It Moves came out on April 2, 2009 (after the March 2009 date mentioned by Anto). I have beat the game, and logged probably 10-20 hours playing it. I believe that the hours on record displayed in the past was correct, but as you can see, it is not correct any more.

You also have to take into account the fact that since the "Total playtime" feature has been introduce about two years ago, it didn't count all the hours you've been playing before. According to this article, gameplay hours are recording since March 2009 exactly. For instance, I've been playing CS 1.6 A LOT several years ago, and I only have something like 30 hours "on record" (because I don't play it anymore).

You can read players reacting about this over here: http://www.wegame.com/forums/general-gaming-discussion/steam-now-reports-all-time-play-time//#post-439405

Steam now reports all-time play time Finally! The stats seem to have started their count about a year ago, so it's not all-all-time, but it's a welcome addition nonetheless. Other programs like Xfire (and even WeGame!) have recorded and reported this info for quite some time.

I know, it can be kinda depressing for a hardcore gamer ;)

The hours on steam is only logged if you are playing while connected to the steam network. If you are playing while steam is offline or steam somehow loses connection, no hours will be logged. If your game is modded in some way and launched via a third party exe, it will not be counted. (For example, with Oblivion, I launched it with the Oblivion Script Extender and I have almost no hours logged).

For some other games, even though you've exited a game, the background process remains because it didn't shut down cleanly. In those cases, steam may even pad your game playing time more than you really played. This can be done multiple times where you end up with more hours of game play in a 2 week span than it is physically possible.

And lastly, Steam didn't track your hours played until a few years ago so it didn't keep track of your HL2 hours when it was first launched.