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It's not that big of a secret. It's based on sales of print copies from certain stores turning in numbers. One of the reasons authors push so hard during the initial week of sales is because the sales matter more. If you google it, you'll find lots of info. One of the excuses publishers used against ebooks was that those books didn't count in the various "best seller lists" and an ebook sale could "rob" the book of its proper place.
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