This issue was recently presented to me and I could not really find a way to resolve it. The person asking the question was making a CreateClampedUniformNurbs crv from 3 pts, modifying the center control pt's weight with a slider. By using the same points, and using the same cp weight but negative, resulted in the completion of the positive curves ellipsoid.
The crv with the positive CP bakes and is selectable in rhino while the crv with the negative CP can be baked but not selected in Rhino. They can both be joined, but you can only select the baked joined curve, if you select the segment defined by the positive CP crv. In GH it can also be revolved, but again, when baked, only the area defined by the positive control point can be selected, even though it is one surface, the area defined by the negative CP cannot be selected. Why is this the case?