Why is it some information supplied by manufacturers doesn't make any bit of sense whatsoever, and just causes more confusion for the end user?
I'll give you a prime example - a nice PSU from xigmatek linked below
http://www.xigmatek.com/product/power-nrpmc851.php#3
Now, note the following stated on the specifications on that page ;
Cooling System :: 140mm Fan,
2300RPM ± 10%
Noise :: 16 dBA at
1300RPM
Now I know that 2300 rpm - 10% is never ever going to be near 1300 rpm.
So what is the actual noise levels of this product when it is in use with the fan running within the 10% margin of the 2300 rpm speed ?
It's like saying that the PSU operates at 0 db ... when it's switched off that is.