I assumed it was sales at the shops, but presumably if pirated versions got into retail channels they would not be counted, since the publisher and legitimate distributors would not have any record of them.
Except for the name of the game, I think I've repeated most of the original post. The game was by a small developer who had initially thought sales of the game might not justify an expansion, but they went ahead anyway (for sentimental reasons, rather than financial).
