Why do severe duty clutch fan always ROAR?

Severe duty clutches will roar all the time, they are designed to move more air.
That's what I don't get, disengaged the severe turns the same as the standard, but it doesn't if you ask anyone that's had a severe fan clutch, they always roar sucking up power and milage.

My 1995 Jeep Grand Cherokee 4.0L has a new single row aluminum radiator, super stant 180*, new water pump and thermostat housing.
 
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