This is a special kind of missable achievement.
You obtain the achievement when you complete de cup and the garbage proof, but if you continuing playing through, the own game takes the achievement away a moment later.
You can check the Steam overlay (shift+tab) and verify that you have the game with 100% achievements earned, but when you click "View All Achievements" and see it in the browser, it's showing "0/1 (0%)".
To avoid it, as soon as you get the achievement go to the Steam overlay, then go to desktop, bring up the task manager and end The Stanley Parable Demo.
Now you can't play game ever again, but you definitely have the game with 100% achievements earned.