Mechanism of hygrometer test

[StringJunky]

I bought a very cheap digital hygrometer and used this method to check it:  https://www.wikihow.com/Test-a-Hygrometer . Can someone explain how the salt/water slurry regulates atmosphere within the bag to this specific figure of 75%?  My device measured 76%... close enough. I mixed it to a consistency of snow.

 

Here's my test setup. It's actually reading 75% now. If you move the apparatus the measured humidity changes a lot then settles back to 75%

 

Edited June 13, 2019 by StringJunky