If not, that could be the answer to your question. The hygrometer you have may not be calibrated properly and is reading on the low end. Most cheap analog gauges are that: cheap. Swap it out for 1-2 quality digital indoor/outdoor guages and you'll probably get a more accurate reading.
No sarcasm, it was pretty cheap and most of all, it's analog.
I'm gonna try it in a closed tub with a fogger and see what the numbers say.
Thx