Apparently bookstores get paid for the better placements too, such as big displays on tables up front and the like. (And the big promos with huge piles of books and posters all over the place - paid for by the publisher.)
Small press can't match that, so it's two copies on the shelves down the back somewhere, and here, you can have them back at the end of the month.
Now imagine were we'd be without the internet and online retailers. I still remember those days with total despair, and will take the current situation with ebooks + AMS ads any day. To be able to publish from my house in the suburbs of Perth, Western Australia, and reach readers all over the world - that's just awesome. 20 years ago it was a futuristic dream.