And here's a more close up example from your own shape.

If we examine just these 2 surfaces here:





If we get in there closely and examine them with a high resolution mesh, you can see that they are not actually intersecting in a clean way, their edges do not actually cross over one another, one's edge is somewhat to the inside of the other:




That's because at the part where they intersect, the sweeps are bending away from each other following different curved 3D paths instead of staying on the same level as each other as they would in a planar path.

- Michael