The front duct is equipped with a METU flange and the one in the back is not. The latter lacks stability and is, for that reason, deformed because of its own weight. Imagine how difficult it would be to connect it to another duct segment!
Ducts fitted with METU circular flanges retain their circular shape, are easier and faster to assemble thanks to joining clips, and offer a higher resistance to negative pressures (*).
This is why we recommend using flanges, especially with duct diameters above 500 mm. The ducts on the attached photograph have a diameter of 1750 mm.
(*) Thease features do not apply to the LF flanges and MU + MUX overcollars.