The problem is that Revit face does not map well to Grasshopper Surface.
Revit faces may have more than one external loop, while not in Grasshopper.
So the conversion to Brep gives you a Brep/Polysurface with all the surfaces included in the Revit Face, while the Surface gives you an untrimmed Surface that overlap with the original Revit face and have same parameterization.
Yeah figure out the BREP after trying every nodes. I suggest creating a seperate node with better naming in new updates, cuz this features is quite hidden