For the past 50 years, engineers have been “designing” duct systems. Quite often, they were not actually designing the systems, but were sizing duct the duct according to some rule of thumb.
The rules varied by different engineering firms and often ducts were sized to be round by a predetermined friction rate that was chosen by the engineering firm. Then if the spiral duct did not fit and sometimes when it does, it was often converted to rectangular, making it a lot less efficient system. Often, the whole ductwork system is “sized” that way using a duct calculator. Not much thought is given to the fittings used and often leakage of the system is not considered during the design phase.