Fanexpert sets a absolute lower limit for some of the case fans whereas your fan may be able to go lower. You can change that by changing the maxdutytokeepstill. What I do is calibrate the fans and set this to a little above the lowest measured % where the fan measured a non-zero rpm (mindutytokeeprunning).
You have to restart the fanexpert service after making changes for them to take effect.
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