I just did a salt test on one of my hygrometers, I did the test for over a day, what I noticed i was the hygrometer would stay between 70-71, so I know my hygrometer is 4 to 5 points low.
The changes in the raedings during your test could be the result, of temperature changes, water to salt ratio, size of container your using to test in, etc....
I dont stress over this type of stuff, I use Beads, salt test my hygrometers, and if I'm off 1 or 2 percent it's no big deal.