Indeed, although it is in no way linear. Also a game that just came out is harder to guage interest for and so they likely overestimated demand in that case, is different for games that have been out for a while. Take another game that was in higher demand than predicted and you would have a very slim difference between shipped and sold. Id say for any single game you probable want at very most a million sitting on store shelves, after first couple weeks of release.