Realistically how do people keep track of this stuff anyway, checking store purchase from over 60 years ago? Come into my house and count how many MJ CDs I got.
In some cases, "sales" were actually product shipped to stores, not records actually sold. Unsold stock returned to the labels might still be part of the "sales", since it was originally shipped out. Sales did not generally count cutouts (records/tapes with a slash or hole punched in the cover sold for cheap), record clubs (10 albums for a penny!), street tapes (basically bootleg cassettes & 8-tracks that were sold at flea markets or parking lots), or promo copies that were sold in some record stores & nightclubs. Although the promos had "not for sale" stamped on them, lol. Sometimes the companies that distributed the stock to stores would keep some for themselves and sell them and they would get all of the money and not the labels and/or stores. Especially with mafia run distribution. Decades ago, people also sold bootlegs of concerts they recorded or unreleased songs by artists. One popular bootleg is called
A Toot & A Snore In '74, which was a recording session with Paul McCartney, John Lennon, Stevie Wonder & a few other artists. Record stores also sold "interview discs", which were records given to radio stations. I don't think stuff like multi-artist compilations like K-Tel, Pickwick, &
That's What I Call Music or songs on movie soundtracks is counted with an artist total sales. Except when it's a one artist soundtrack like
Superfly (Curtis Mayfield) &
Purple Rain (Prince). If a particular artist was a member of multiple bands (Eric Clapton, Steve Winwood, George Harrison), each band is counted separately and not with their solo records.
Sales were also often faked before Soundscan. Payola was not just for radio airplay. Labels would also sometimes pay record stores to report to Billboard that a record sold more or less than it really did. The RIAA does not audit sales, they just go by what the labels tell them and reporting sales in voluntary. That's a reason that when Soundscan came into being, the charts totally changed. But not all stores had the Soundscan equipment, usually small mom & pop stores didn't have it. Places like Abu Dhabi, Russia, & China had black market records & tapes, which are not official. When CD burners came out, people would burn CDs & DVDs and sell them at their jobs & in their neighborhoods. Counting all of these sources, nobody really knows how much any record sold. Also for those older acts, total sales are not just albums, but physical singles too.