Amazon Author Central's estimated sales figures are provided by BookScan, which is run by Nielson (the same company that tracks television ratings). Like television ratings, BookScan does not query every bookseller, retailer, or even publisher to produce its data. It relies on point of sale data from certain booksellers to generate an estimate. Your Gatekeeper Dashboard displays sales from every single bookseller and is 100% accurate.

You can read more about NPD BookScan from Author Central.