Maybe your BIOS is controlling it, and will only make it faster when a certain temperature is reached. This could mean two things, your cpu is nice and cool, or your BIOS will speed it up once you hit 50 C or something, what are your temperatures?
How big is the fan? My 12cm fan never goes over 900 RPM as well. The bigger the fan the slower it will spin, and therefore less noise, that's why I think bigger fans are better than smaller noisier fans that spin at a higher RPM.
Yeah, if your CPU is nice and cool, why would you want the fan to rev up? The slower the better because, the slower it goes the more silent it will be.