couple things that could be doing it...

one.. are you "smoothing" in Hex?  this causes the program (both Hex and carrara) to Add polys at export to make an item look like it does inside that program.  make since?

also.. when exporting and importing there are options... I believe there is some advanced ones in Hex... not positive but maybe look around there

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Hexagon, Carrara, Sculptris, and recently Sketchup.