OK - Sorry. I just recalled that some people have had PWM issues with some Artic fans. I don't know why you are seeing lower RPM for the fans when they are plugged into a port on an Aquaero 6XT or D5 Next vs when they are plugged into a PWM port on your motherboard.. Perhaps someone from Aquacomputer will chime in here. Good luck.
Yep no worries, This is the reason I am wondering if some sort of filtering/smoothing is happening.
I know usually when you apply smoothing to let's say something like a vehicle dyno graph the more smoothing you apply to the X axis the smoother the curve looks but the lower the numbers will generally be over the X axis.
I wonder if something like that is being applied to the fans.
Honestly it has to be because if you ever have watched a fan on a fan header on something like a motherboard the RPM will fluctuate a bit even if the PWM signal is steady but if you look at the fan graph over time from a fan connected to an aquaero it is perfectly smooth 0 RPM deviations.