Accepting everything you said, I would simply tack on that I think what you call "Southern 6b" shows an amazing plant palette for the following reasons -- all theory from my head, not a book or other source: First, as a practical matter, if you were a living thing outdoors in Nashville, as opposed to, say, Harrisburg, PA, you would notice that Harrisburg is colder. The sun angle is lower, daily highs and lows would average out colder than Nashville. This would help explain the presence of plants in Nashville that we associate with warmer places, as well as their absence in Harrisburg. But the great equalizer in determining the zone designation is the winter minima, not average lows. And in this regard, what Nashville has working against it in winter is its distance from "big water" and it's location on the western side of the Appalacians. An arctic airmass plunging down from Canada or the northern plains is going to be just a little more moderated, usually, by the time it hits Harrisburg than it might be when it hits Nashville. Hence, the typical winter early morning in Nashville is warmer than Harrisburg, but on zone-defining mornings, Nashville can get every bit as cold as Harrisburg gets. And that's the criteria for these zone maps.
The comparison works for 7a as well. Wilmington, Delaware is 7a. So is Knoxville, Tennessee. But I'd much rather try to grow marginals in Knoxville than Wilmington. Still, there can be nasty winter minimae in Knoxville, exacerbated by Knoxville's altitude and proximity to even higher altitude mountains.
NB: The zones attributed to the locations in this post are taken from the 1990 ("cold") USDA map. Add a half zone for the 2012 map. Doesn't affect the general gist of the post.