Accuracy of salt test calibration

I'm a newbie so please forgive me if this is so basic as to belong in the newbie section.

I performed the salt test as directed but only left everything in the bag for 12 hours as opposed to the 24 hours I've seen suggested in some places. My digital hygro registered 65%.

However, twice I've left the hygro outside over night and both time it was dead on with the local weather station's RH reading. What gives? Did I not perform the salt test long enough? Are different results to be expected when leaving the hygro outside?

This whole hygro accuracy/humidification level is necessary but it sure takes a lot of the fun out of this hobby.

HELP (and thank you, too)!