problem when calibrating hygrometers

[Vikder]

I've got a weird thing happening when I try to calibrate my hygrometers. I'm curious if anyone can help me figure this out.
I put my three hygrometers into a clear Tupperware container with the proper salt and water mix. After 24 hours here is what is read:

Hygrometer #1=72%
Hygrometer #2=73%
Hygrometer #3=78%

Ok. By the above hygro #1 is 3% under, #2 is 2% under and #3 is 3% over. Cool, no problem.
Here is the issue. When I put each one separately into the Tupperware here are the results after 24 hours.

Hygrometer #1=74%
Hygrometer #2=76%
Hygrometer #3=80%

Now how the heck can this be? This just makes no sense at all. I have even repeated the above results by doing the tests 3 times each! Any thoughts on this?

Thanks,
Vic