Eh, sales are just classic market segmentation. By periodically dropping the price of the game towards some minimally profitable number, they can divide the market based on how much you're willing to pay. People willing to pay the "full price" will do so in the first day/week/month, and then people who are willing to pay less and less will do so more and more months later.

To put it another way, they never planned to sell 10 million units at $60. They planned to sell 1 million units at $60, 2 million units at $30, 3 million units at $20, and 4 million units at $10. Price drops are the only effective and legal way to do that.