A duct section having larger size than the preceding section even though delivering less flow rate

I have carried out duct sizing for a Mechanical ventilation system with six supply outlets each having 0.2 m3/s flow rate. The ductwork is largest close to the system marker (AHU) and decreases downstream of the flow as would be expected. The expectation is for the duct size to keep on decreasing or at least stay the same as the flow rate decreases (due to subsequent duct sections serving less number of supply outlets). However the result showed an increase in duct size as well as a negative pressure drop at a particular section which does not seem right. Could anyone please explain. Thanks