That's the problem - balancing ductwork is a lost art. Any more, they just bang the things together, and to make it worse, even if a building was close when it was built, if they remodel or replace the furnace, they never re-balance. Apologies to any HVAC contractors out there - but I think these days you're as likely to find a seamstress who knows how to card wool as you are to find a HVAC contractor who knows how to balance ducts.