They will yield different values if and only if you have subsurfaces on the surface, that is windows, doors, skylights, etc.

The surface.grossArea is the area of the surface itself. The surface.netArea is the surface.grossArea minus the surface of any subsurface. That's best explained with a little picture:

So if here in the case of the left surface, there's a subsurface - a window - so they yield different results. On the right, there's no subsurface so they give the same value.

Note that surface.netArea will NOT calculate the thickness of walls or anything like this, there is no thickness in E+!