The ingredient "salt" can mean many things. That label misleading people. Look at the description here:
Kosher Salt
It says "It even contains 53% less sodium by volume compared to table salt".
So that is not 100% sodium chloride which is needed for the salt test to get 75% relative humidity...
You should calibrate it to see how far it is off. I may be close or it may be 15% off. You won't know until you test it. I've seen them close and way off with the same brand.
1 tablespoon sodium chloride, 1 teaspoon water is the right ratio.
What exact salt did you use?
Did you use an actual...