I believe I have seen the phenomena that you are referring to. It is actually the crystalline state of the material itself. I noticed this when turning parts from 1.5" diameter extruded 6061-T6. I do not know if all sizes of this material exhibit the same condition, but only that one job seemed to really have an exaggerated effect this way. I had to buff the surface with scotch-brite to try to blend and hide it.