Cumulus Support

I've had my WH1091 for just a week and am quite pleased with it, and very pleased with Cumulus.
The indoor humidity reading however, is consistently low by about 10-12 percentage points, when I compare it to a sling psychrometer and I'm not able to calibrate it -only the outdoor humidity sensor. Any comment?
cheers
Ned

I suppose I could add a calibration facility for indoor humidity; I've never really given much priority to indoor temp and humidity, given that they are not generally directly relevant to the weather.