The answer to that question is found in the width of the doorways.  Because the duPonts installed doorways in the Drawing Room that were wider than the ca. 1812 doorways in order to fit the grander scale of the room, the trim that framed the top of the ca. 1812 doorways could not be reused.  Therefore, only the lower portion of the Madison trim was used on the first floor and the duPonts’ carpenters built a set of new, wider arched trim to fit the top of the doorways.  This process was reversed on the second floor because the doorways were narrower to fit the smaller dimensions of the second floor spaces.  This meant that while the top portions of ca. 1812 arched doorway trim were too wide for the first floor, they were a perfect fit for the second.