Economics in gaming II on the GalCiv 2 Forums (thanks Frans) has more on initial sales of Galactic Civilizations II: Dread Lords, discussing direct sales versus retail sales, offering some insight into the formulas used to determine how many copies of a game to stock and restock through its retail life-cycle:

Overall sales of a product is a function of Initial Sell In X Marketing X Game Quality.

When it's time to reorder games to replace sold units, computer algorithms come into play. Each retailer has their own system for predicting how many units they will need. But it roughly boils down to something like this:

The first week's sales of a game = The subsequent month's sales of the game = the Subsequent 3 months sales of the game = The subsequent 6 months sales of the game = The subsequent year's sales of the game.

Therefore, if you blow your first week sales numbers, you're in big trouble. And that's where sell-in comes into play. If you can get 100,000 units on the shelf first day then you can assure very high numbers of sales.

Galactic Civilizations II's initial sell in was much higher than the original's -- 3X as much (but still only a fraction of the roll out of a huge title). And sales have been unexpectedly strong. But because we're a smaller player, our ability to get units onto the shelves is much less than a larger publisher. We can TRY to get our title out into all retail chains on Day 1 of release but as we've learned (The hard way along with our customers) our ability to actually get....the...units...in....their hands and up on shelves is more problematic.

Hence, we have cases where some stores -- even in the same retail chain -- have the box on the shelves while others do not. Which drives us nuts.