Cooler (Noctua D15) has fans connected to CPU_FAN and CPU_OPT headers. However, they spin at different speeds, why?

phemark

Honorable
Nov 23, 2012
19
0
10,520
Hi,
Basically, at low temps, one is spinning at 700 another at 400, but both reach 1500 when at high temp. Why dont they both spin at same RPM at low temp?
(Asus z170 pro gaming motherboard has only one control for CPU in its BIOS)