Extracting longest surface edges

I have a clump of components that takes a ribbony rectangular surface and finds the 2 longest edges. I need to do this over and over, in several definitions, so I thought I'd write a C# script to replace the whole group (to keep things tidy and easy to update).

I just got started with it yesterday, and rapidly realized that I'll have to go deeper into unknown C# and RhinoCommon territory than I really want to. So I'll probably stick with my clump, for now.

But I did run into something I'd like to understand, for my own education: in the enclosed example, for the given surface, the BrepEdges component finds 4 naked edges (as I'd expect), but my C# script only finds 3 edges. Why?

Thanks for any help understanding this...