A "term" that helped me understand it was a circular in MoI is also called a "rolling ball" fillet. So the fillet will be applied as if you have attached a sphere to the edge and rolled it along, wiping out whatever it touches (Or "ADDING" is the more complex part). The constant distance is an exact, "parallel edge" surface (The edges of the fillet are always equal).

Not a new explanation. Just different.