While the left ball is a real sphere primitive, the middle and the right balls are polygon meshes. The one in the middle has the ROUNDED attribute, which is first supported with release 0.46.
In the Docs-Section I statet, that things like phong-interpolation would not be possible with WinOSi's rendering concept, and real curved surfaces (like the left sphere) have to be used for smooth appearance.
Nevertheless, it turned out, that 5 lines of additional code make a very good fake of a smooth surface, but it is not free of some artefacts except the polygonal contour-edges of course: You can see a dark face at the bottom right side of the middle ball, which is hit by so few photons directly, that illuminance interpolation fails here.

This works with specular reflection too: