May be thats the reason.yes i think the same .. but i guess the RPMs on the FAN ( if tht can be measured) are lower than reference cards .... another thing is that the cooling design is compeltely different.. as with other 5850z this doesnt throw the hot air from the rear of the card .. making it sound like a wind turbine ...... it dissipates it along the sides .. making it super silenttt ... i mean my 25 db 120mm fan can be heard but not this ...
In-fact, cooler with shroud helps in keeping temperatures at low levels.
If you'll add a card board shroud over this cooler(leaving fan space open only), temperatures will lower by at-least 5C.