"More than 9,700,000 copies have been sold in the United States since 1991, when Nielsen SoundScan began tracking sales. Since its release, the album has scanned more than 8 million copies in the US"
Sorry, I know this is really more a "music business" question than a Metallica question, but honestly I thought I'd be more likely to get an accurate response here. I know modern charts use "album equivalent sales" and whatnot due to streaming, but what the hell does it mean that it "sold 9.7m copies" but also "scanned 8m copies"?
SoundScan began tracking sales data for Nielsen on March 1, 1991 so since march 1, 1991 8 million copies were scanned the 1.7 million is from the albums release to march 1, 1991.
SoundScan tracks sales by the actual barcode. The Black Album is the #1 album during that entire “SoundScan era”, which started after AJFA. The difference in the numbers you have are before & after SoundScan started tracking it.
I'm pretty sure the difference is that scanning refers to when the item has been purchased at a retailer with a trackable point of sale system of some type. The scan itself refers to scanning the barcode to register the purchase of the product. Of course there are other ways (like merch booths) to sell the product that don't always have a POS system so the delta between the total copies sold and the number scanned is made up that way. At least I think!