I'm not sure, but by just assigning a fan, you may be getting a fixed flowrate. Or, if by assigning the fan, you may be getting the same fan curve that you're entering and that's why you're getting similar results.
That's where my confusion lies. Fans in the engineering database or fans you define all have fan curves associated with them anyway so you're getting the same flow rate whether you pick the fan or the curve.
The only possibility I could see would be whether or not the fan curve accounts for the swirling airflow of an axial fan, or if the regular fan accounts for this at all.
I noticed that you can specify rotating air flow only when a fan curve is selected. Doesn't work when you just use the fan.