One case fan spinning faster than others

[redpanhead]

I have 6 case fans, 4 spinning roughly 450 rpm.
another one spinning around 1050 rpm and the last around 950 rpm.
why these last two spin twice as fast as the others.
I'm using a Asus maximus hero VIII and also corsair link with a h115i cooler.
When I try to optimize these fans in the bios, systems locks up . bios also wont let me set fan mode on either pwm or dc mode. these are all corsair 3 pin fans

 

[AppleGwava]

Are they all the same model?Airflow and speed fans have different minimun speeds which cannot be surpassed

 

[nukemaster]

The board may have different speed settings for case vs cpu headers. I know my board does.

As for the 950 and 1050. This may be a variation in the fans. They are normally rated at +/- 10%.

If the board is controlling 3 pin fans, it is already using DC and not PWM.

 

[faalin]

Why dont you use the AI suite 3 software to control the fans instead of the bios.