The fan shroud was made 1.8 and 3mm longer because with the Volcano nozzle mod (https://www.printables.com/model/656891-qidi-x-max-3-volcano-nozzle-adapter), some prints with high cooling failed with the error “the temperature of the nozzle is too low” and the cooling was not good (see photo of the object [printed with original 360° part cooler duct]). I have only tested the 3mm longer version, the 1.8mm version is untested.
The author remixed this model.
The fan shroud was made 1.8/3mm longer for the Volcano nozzle mod.