Hi Guy,

re:
> Maybe what I'm seeing is some sort of shading illusion or just the curve of the helix.

Well the surface isn't flat it does have a curved shape but it's normal for a shape made up entirely of lines to have a curved silhouette like that:





It's similar to the silhouette of a hyperbolic paraboloid surface:
https://en.wikipedia.org/wiki/Paraboloid#Hyperbolic_paraboloid

You'll get a curved shape like that if you traced a curve on the surface at an oblique angle instead of in the locally linear direction.

It doesn't mean there is a bulge in the surface though.

For how to avoid it, I don't think you can avoid it if you want to have a helical shape, it's just fundamental to that type of surface.

- Michael