Two DNA Circles or a Venn Diagram?

Ancestry updated their DNA interface by eliminating a large number of false positive matches, and adding a new feature called DNA Circles.

You can read more here and here on Ancestry’s Blog

With some initial reviews at

• The Legal Genealogist 
• Genea-Musings 
• The Genetic Genealogist

What are DNA circles? According to Ancestry:

Ancestry told me that I am part of two DNA Circles
One has 8 descendants of Israel Swayze, Jr
One has 5 descendants of Abigail (Coleman) Swayze

Ancestry calculates the likelihood of the members in the circle sharing the same shared ancestor: Strong, Good, Some, Emerging or Weak. In all of my cases, they're 'Emerging.' This suggests to me that we're all in these circles based mostly on our Family Trees, and less on our DNA, or else it would probably be stronger. However, their description of DNA circles above states all members must share some DNA with at least one other member of the circle. 

Israel and Abigail were husband and wife, and I am unaware of any other spouse for either of them.

All five matches in Abigail's circle are in Israel's circle.
According to our Online Trees, my most recent common ancestor for the three in Israel's Circle that aren't in Abigail's is actually Israel and Abigail's daughter, Johanna Swayze.

I have no clue why these two circles aren't identical - or how Ancestry would be able to distinguish whether particular DNA for anyone in these two circles was from Israel or Abigail, since we all, if the research that went into our online trees is accurate for all of us, are descendants of both of them -- unless Ancestry had information about DNA circles involving other Swayzes or Colemans. If this is the case, I would like to know this information.

Another conundrum is that a known second cousin, a descendant of Melvin Elijah Van Every through a different daughter, isn't in either of these circles, even though her tree is public. I wonder if this implies she doesn't share any of the Swayze-Coleman DNA. Perhaps she does have DNA from Israel and Abigail, but it's not the same DNA that the eight in these circles inherited.