It’s probably because the game’s price was lowered from 17.99 to 11.99, and some of the store clerks simply overlooked the changing of price tags. Used games move in and out of those stores so quickly that it would be very difficult to keep up with all of the pricing changes. I imagine if you were to take a $17.99 copy up to the register, you’d find that it was $11.99, and just mis-labeled.