According to the Technical Product Spec the PWM offset doesn't do anything except when Quiet Fan Idle Mode is turned on. I haven't messed with it much, but it was set to 100% when I got the board and turning it to 0 didn't change anything with regard to fan RPM that I saw.
The Intel board uses a closed loop thermal control for each cooling domain. Meaning it allows temps to rise to a certain point, and then will take action to stop them from rising further. This is the first motherboard I've ever used that does that. Maybe it's normal for enterprise / server stuff. I've had very little exposure to it so I can't really say.
There is a baseline fan idle speed that is chosen based on the ambient temperature, altitude setting, and whether you picked Acoustic or Performance in the BIOS. The max temp it will let a cooling domain rise to is set by thermal margin clamp values. You can "hack" the .SDR file to control both idle fan speed and the clamp values. I lowered certain thermal clamp values by 15C and now it stays in the mid 60's instead of the low 80's. I left the idle alone since it's running pretty close to the minimum PWM already and my basement is even cooler than where I was testing.
I will probably lower the thermal margins by another 5C, replace the thermal compound with some top tier non metal stuff (Thermal Grizzly Hydronaut), and make sure the heatsinks are actually flat and smooth on the bottom and run it like that for a while.