I wonder how you are measuring it ? This cheap dial hygrometers ( human hair based ! ) are accurate plus or minus 100% I think ! They are not far out within a few days of being reset, but if longer than that, of no value. Reset by wrapping in a wet towel for a couple of hours, then adjust the needle to be on the 100% mark exactly.
The old-fashioned and very reliable way is a wet and dry thermometer, but they are a nuisance to remember to keep filling up the little bottle with very pure water, and I have found the wick rots, and again, they are then unreliable.
Currently I use a little hand-held digital one, giving an instant read-out. According to my local Mercedes dealer, using one to check out the climate control system in my car, they are the best.