They dont, your right.
But if you want to use an Acurite like Chris said that you fear isn't accurate, you do this test.
When I used to do this, I'd calibrate them and then put a plus or minus on the front of the Acurite with a Sharpie the % of difference it was from 75%. So if I peeked in a cage and it said, 60% and then had a +5, i'd know it was really 65%.