Thanks to everyone for their comments.

Hi BurrMan,
The lack of tangency is a major fault.
I had assumed all along that the ends were vertically tangent, and had written so in some preliminary documentation.
The point density at the ends seems low...

Will do some checking...

- Brian

Trimming the top and bottom curves at an endpoint with a tiny circle, and Blending, makes the ends tangent. But moves the endpoint a bit...

The biconcave script does not have this lack of tangency problem...