On Rhino a “sphere” is a rectangular surface (topologically speaking) defined by 45 control-points, with 2 edges collapsed into poles and the other 2 edges being coincident (the seam). Then there are the correct “weights” for each control-points.
While you launch the command “sphere” Rhino have a function that works with just a center point (or better a plane system) and radius, but after the function is executed you have a “complex” surface, not a “virtually perfect sphere”.
Rhino don’t store the information of how was an object made. To know if a surface is a sphere you need to create/use a method where all its points and weights are checked again (with a tolerance).
Grasshopper, instead, can store the source of a sphere (origin+radius) to keep it “pure”, rhino cannot.
(Actually grasshopper re-create the spherical surface every time, the same way as rhino. Rhino is a surface modeler. )
You could want to “intercept” the rhino command and transform it into grasshopper elements.
Why don’t you make the sphere directly on grasshopper?