in my experience; i have set the hygrometer at 75% after a good salt test. i have seen hygrometers from the factory be as much as 15% off after the set. always set it the first time; and then retest it after the humidity level drops. usually wait about an hour.
it should test out at around 75%. if it doesn't; take it back and get another one. it is the LIFEBLOOD of proper storage.
Make sure the container your using in airtight.
If not it can effect the reading.
Leave it ver night and see what you got.
If the hygrometer does not stay stable, take the advise above
& get a replacement!