I think it is to do with backface cull on object creation option. Backface Cull on Object Creation Determines whether to display faces with normals pointing away from view. When turned on, you see through the wireframe to the backfaces. This option applies to wireframe viewport displays only. In most cases, you'll want this item turned on. However, if you're modeling with NURBS surfaces, which consist of single-sided planes, it's easier to view them from all angles when backface culling is turned off. This control affects only the created objects, and you can reverse the effect on each object by changing the Backface Cull setting in the Object Properties dialog for that object.You might turn off Backface Cull On Object Creation before creating your NURBS, and then turn it on again when your finished. You can globally change the display of backface culling in the viewports by turning on Force 2-Sided on the Rendering Method panel of the Viewport Configuration dialog.