Curvature of SubD contours

SubD, for me, so far, has been producing perfect or near-perfect zebras.

When I learned recently that most of the Curves from Objects toolbar worked with SubD objects, I decided to start playing around with various contours and checking them with _CurvatureGraph.

Here are some results:

For duplicated edges (magenta in this document), very good, simple curvature:


The same is true on more complex edges:

But when I obtain a contour with _Intersect using a plane set to bisect faces directly between edges (cyan curves), things get weirder:


(oddly the smoothest curvature here is on Penta faces)

Results are identical after converting to NURBS polysurface.

Is this an expected result of how SubD math works? Or is this happening because SubD is still a work in progress?