Do you have a temp gun? Is the Acu-Rite new? Have you checked the Acu-Rite against a temperature that you are pretty sure of?

I use Herpstat's and have the larger RBI cages with RHP's and the cage temp correlates pretty well with the Herpstat setting. Lets see.... hang on....

Okay, I have my Herpstat set almost 2 degrees lower than where I want my cage to be