Actually, it's not as simple as you state.

  The normal modifier DOES indeed affect the shadow line of an object (ie.
the line that is between the part of the surface that faces the light source
and the part that faces the opposite direction, ie. the line where the
shadowed part of the object starts).
  To see how the normal modifier affects this line, try making your object


  What happens is that the sphere surface shadows itself. When a point is
in the "dark side" of the sphere but due to normal perturbation the normal
vector of that point faces the light source, there's a surface between that
point and the light source and thus it gets shadowed.
  This can be fixed by making the object shadowless, but then you lose all
shadows from that object, which is usually not wanted.

  I have suggested some work-arounds for this problem long ago, but no-one
seems interested enough to make any.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):_;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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