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quoted from steampowered.com
“Missing Stats
The Steam development team is still ironing out some of the bugs with the Steam stats and achievements.
If you notice that stats or achievements do not appear to be tracked, or previously attained stats and achievements are missing, that is a good sign that either stat tracking is temporarily down or that you may have an Internet connection issue. During this time, your stats and achievements may not be recorded. Unfortunately, that means they will not appear later when everything returns back to normal.
Occasionally there may be issues with attaining certain achievements – in these cases, you will need to wait for a patch addressing this problem in order to attain them.
Very rarely certain achievements will be reset or adjusted, typically following a patch that is intended to fix issues with that achievement.
In any cases where you are missing achievements, they unfortunately cannot be restored. You will need to re-attain achievements that have been lost.”