box primitive

this is admittedly an inane qeustion, but i was wondering why the geometry primitive box(float xdim, float ydim, float zdim, Appearance a) is set up to construct a box that is actually double all the purported dims in size. the api just says it “constructs a box of a given dimension and appearance”. i know the no argument constructor has everything centered…with one unit on either side of the origin and so this is presumably in line with that…still this had me confused until i labeled the axes and realized what was up.