I don’t look at it so much as a canard than I do a different interpretation of census records from 1860.
Here’s a good resource from someone who’s gone through that data.
http://fisher.lib.virginia.edu/cgi-local/censusbin/census/cen.pl?year=860
The 6% comes from number of slaves and number of southerners calculation as a whole. The author says this is specious because not all southerners had the means to own slaves. He also notes the age and sex groups. He breaks the southern population into age and sex categories and then comes up with numbers for some states like you cite. MS being the worst at 49% - all this on straight calculation of 1 slave per southerner.
You can Google or Bing all over the web and come up with 5%, 6%, 21%, 25-30 (from “From Slavery to Freedom”, to the numbers cited by the link I provided.
I don’t think we’ll ever know concisely for a number of reasons:
1) This is a question founded on political and ideological beliefs and data can be manipulated thusly,
2) Places like Wikipedia (in various forms) provide ‘answers’ all over the place (from 6% to 30%) - the ‘peoples’ dictionary, and
3) Many organizations that have been through the 1860 US Census data patently recognize the data are somewhat incomplete, brought on by various events around that time (e.g., secession, etc in 1861).
All I know for sure is that no one in my family which I have traced back to that era and before ever owned slaves - they were mainly too poor and were subsistence farmers at best.
I think it's important to understand this whole "how many owned slaves" question realistically.
The usual answer is "very few owned a lot of slaves", overall in 1860 only 5% of eight million Southern whites owned slaves, and of those only a few thousands owned 100 slaves or more.
But if you ask, "how many whites lived in slave-holding families?" -- then the answer is quite different, and needs to be understood from the perspective of Deep South compared to Upper South and Border States.
Seven states of the Deep South (South Carolina to Texas) included 2.5 million whites with 2.3 million slaves.
Whites lived in 677,000 families of which 181,000 owned slaves = 27% of white families owned an average of 13 slaves in the Deep South.
Four Upper South states (Virginia to Arkansas) included 2.9 million whites with 1.2 million slaves.
Whites lived in 668,000 families of whom 133,000 owned slaves = 20% of white families owned an average of 9 slaves in the Upper South.
Four Border States (Delaware to Missouri) included 2.7 million whites with 427,000 slaves.
Whites lived in 565,000 families of whom 77,335 owned slaves = 14% of white families owned an average of 6 slaves in the Border States.
So, overall the 1860 average is 21% of 1.9 million Southern white families owned on average 10 slaves each.
Yes, these numbers are somewhat different than I've posted before and represent corrections.
The biggest surprise is to learn that Deep South (27%) and Upper South (20%) had somewhat fewer slave-holder families than I'd calculated before.
Border States at 14% overall are slightly higher, ranging from 3% in Delaware to 11% in Maryand & Missouri to 19% in Kentucky.
The numbers help tell the story of why the Confederacy was more successful persuading Deep and Upper South states to join it, than it was in Border States.