Rhino: Fillet and FilletSrf

In Rhino,  using Fillet and FilletSrf on the same edge of a surface model will give you different results.

For example, if I create a cube and use Fillet to add a fillet to an edge, the edge turns into a filleted surface. When I click on this surface, it will be highlighted together with other surfaces of the same cube. It will be like this:

Whereas, if I use Fillet to add a fillet to the same edge of the cube, I click on this surface, it will be highlighted alone. Beside the edge is filleted alone and the adjacent surfaces will not be affected. That also means the newly filleted surface is not part of the surfaces grouped in the cube any more. I may need to regrouped it with other surfaces. It will look like this: