Spherical mapping has discontinuity at poles. SubDs are rendered with fairly high tessellation and therefore discontinuities often show very small artifacts. A planar nurbs surface instead is rendered with a very sparse tessellation and therefore the discontinuity causes a very large artifact. When you convert the SubD into a polysurface the result will not be a single planar nurbs surface and therefore it will often render with denser tessellation which again makes the texture mapping discontinuity artifacts appear smaller.
Rhino’s Rendered and Raytraced display modes do spherical mapping per pixel and therefore tessellation density does not cause issues there.