Purely a guess on my part but it seems that most juice vendors go with:
extra low= 6 or 8mg
low= 10 or 12mg
high= 15 or 18mg
extra high= 24mg
True. But A.V. tobacco juices go from in increments of 6mg from 6 to 24.
If it was a mistake on Porphy's part, I truly think it was an honest mistake. Based on everything I've read he's got a great reputation.