Thanks for the file.  I narrowed the bug down to a logical error in the
function Invert_CSG_Intersection().  Inverting an intersection should
lead to a MERGE object, but instead POV converts it to a UNION.  This
is why you get the extra surfaces.

The fix is as simple as changing one line from
  Object->Methods = &CSG_Union_Methods;
into
  Object->Methods = &CSG_Merge_Methods;

I'll send the fix to the POV team right away.

-Nathan Kopp

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