I think some of the problem with my home is its shape; picture a big, square donut. We've got a large courtyard in the center of the home, which means I have twice the number of exterior walls as a "regular" house. Also, there is no continuous attic between the "public" side of the house - kitchen, dining, laundry, etc. - and the "private" side (bedrooms). I don't think a single set of ductwork would've worked in our case without some significant reworking of the floorplan and/or interior details.
Contractor was "AT" in Tomball.