I made a very simple scene with a Ground Plane and a bouncing Sphere by making the Sphere an RBD Object.

I notice that when the Sphere is a Primitive, it bounces in the Z direction.
NURBS, Bezier, Mesh, and Poly Mesh all bounce perfectly up and down.

I understand why a polygon sphere might bounce sorta erratically, but I don't understand why a sphere primitive wouldn't bounce perfectly.

I think I'm missing a basic understanding of what a primitive is and how/why it acts differently in a simulation.

Can someone please explain this to me?

BTW, I did increase the Bounce for the GroundPlane & Sphere using the Physical Tabs in their respective nodes in the AutoDopNetwork. But I don't think this is what's at issue.