With "Circular" the fillet cross-sectional shape is always an arc of the specified radius, and in areas where the surfaces being filleted change angle to one another it will make either longer or shorter length arcs of one certain radius, like this:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2457.2

With the "Constant distance" type fillet, the fillet rails (the edges on either side of fillet surface) are generated to be a fixed distance value apart from one another, and then an arc is filled in between the surfaces and the radius of that arc will adapt to whatever radius is needed to fill it in, so the arcs that are generated with this method do not all have the same radius but instead have the same chord length between their endpoints. The arc's radius will be larger or smaller depending on the angles between the surfaces at that particular point along the fillet.

- Michael

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.

Thanx understood.