Steam says I have 9 of the 14 achievements in 10,000,000, but only shows me 4 of the remaining achievements and indicates that there is one it is not showing me. What is the hidden achievement and how do I get it?
You currently miss the Freedom achievement, that you would get for "finishing" the game by getting 10,000,000 points in a round, and gaining your freedom by entering the door on the top right. This is the only achievement which is hidden until you get it.
You can see the complete list of achievements to compare.