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You have to keep in mind that Nielsen and the author's earning report are measuring two separate things. One records purchases and the other "sales". It's expected they would reach different conclusions.
Both reports require consistency from year to year and Nielsen/Bookscan, Amazon and AE have adjusted their methodology over the years.
I'm not saying one is right and the other wrong. But you can't compare one to the other or mention them both in the same context.
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