you should calibrate your hydrometer. either buying a kit or doing the salt test.
guess it would be better to tell you what the salt test is:
mix 1 tablespoon of salt with enough distilled water to make it a paste. put the salt on a small dish or lid and place both your hydro and dish in a baggie. leave enough air so that baggie doesn't touch the hydro or salt. let it set over night and it should read 75% humidity. if you can calibrate it to read 75% if note make not of the difference.