As you found in another's study, there were a lot of 'planters' who were absentee planters. They may have come South for the occasional winter but it wasn't their address in the social register. I read somewhere that a few of the large cotton farms (called that because they really didn't have a big house on the place) were owned by Northern mill owners and the cotton was just inventory for their looms. I'm not sure how if or how it can be documented but a lot of the business dealings were handled by factors who were Jewish. Very possibly there were some in the slave trade/sale. Some planters owned more than one plantation too.
Before they started getting almost desperate for troops and they began hauling in the 'reserve' people, the number of slaves exempted one from service if it was a large enough number and the product was needed by the Confederacy. Age also was an issue. Some of the larger plantations were owned by men older than the 'draft' age and some were owned by women. Highly skilled craftsmen such as gunsmiths and a few other areas were thought in the beginning to be of better service doing their regular job. That was the reason one of my GGUncles was allowed to buy out of the CS Army. He paid another and was asked to go home and maintain the ferry service on the river as he had before enlisting.
So, as far as I can tell, too many variables to get the perfect answer but I can almost bet the author (and that's not his main gig) padded his facts and numbers to get the better reaction from those who bother to read his stuff.
Pam