Heat / rack / herpstat question

Hello everyone I have a question regarding heating a rack via heat tape running off a herpstat 1.
I currently have a 6 tub 3 level rack . I taped the probe for the herp to the top level ( the only level currently with my snakes and set the temp to 90. Now when I use my temp gun to check temps it
appears to be spot on ( although the cheap stick on thermometer says it's a bit higher ) the issue im
having is when I check the temps on the lower level it reads much higher ( about 100-105) what would
cause this ? I've read it's best to put the herp probe on the middle level so I plan on doing that and seeing
if it helps . Any suggestions are most welcomed. I just want to get all the temps in the same range before
I move my other snakes over to the rack .

Thanks so much !!!

Temperatures can vary through out heat tape.
This is where you would find the medium temperature on the run and put your probe there and then adjust until all shelves are within a good range and not over 95 degrees.