That's part of your issue. Your thermostat's probe need to be sandwiched between the heatmat and the outside of the enclosure. (See the below thread for pictures)
Think of this way, with your probe on the inside, the mat is having to heat up through a layer of plastic to hit the temp your thermostat is set to, so of course the mat is actually hotter than it really needs to be. Plus there are other issues that arise from having the probe in the enclosure, such as liquids getting on it, your snake moving it, etc etc.
https://ball-pythons.net/forums/show...cement-for-UTH