Sunon 80mm fans rpm=??

Question

I'm using a sunon 80mm fan on a fan adapter on a thermoengine hsf. Motherboard monitor reports the fan RPM as 2350 .... yet the sunon site suggests it should run at 2900... Have I got a faulty fan or is MBM lying ? I've tried changing the fan type and multipler in MBM .... whatever I try it never reads 2900. Thanks in advance -- dkh ... his dancing leg ...