Hyperbolic Surface Offset

I am trying to understand why when I use the offset surface command a get an odd looking surface. If I have a simple hyperbolic surface and use the offset surface command I get the purple surface. I would have expected to see something like the red surface on the right. What am I missing?

Hyperbolic offset Test.3dm (76.5 KB)

Offsetting follows surface normals. Evaluate your surface at its edit points (run Rhino’s “Dir” command to display them) and see what direction the normals point, then you will understand. The red example you show would be the result of a move in the -z direction, not an offset.

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I think I understand now. It would be the equivalent of doing on an offset of the black curve distance of 4, which will give you the red curve. If you move the curve vertically you would get the purple curve.