Are you taking the temperature over the UTH on top of all the substrate, or are you taking the temperature of the glass over the UTH after you've moved the substrate out of the way? You should be taking the temperature of the glass as ball pythons will burrow. Just put a very thin layer (around a quarter inch) of substrate over the glass and put the hide over top.
If your room ambient temp is 71*F the glass tank will leak a lot of heat; glass is a poor insulator and the heat mat does not add a lot to the ambient temperature inside a tank. A heat lamp or ceramic heat emitter (CHE) will increase the ambient. A CHE is more expensive but usually lasts longer and works more efficiently so it's cheaper to run. If you use bulbs buy several because they usually blow at the worst possible time.
Misting an enclosure can also crash the ambient temperature for two reasons. First, tap water can be pretty cold, and second, evaporation uses heat energy so increasing the humidity by misting will cool the air in the tank. You can keep the humidity in the tank by covering the top with aluminum foil - just leave an opening for the CHE or heat lamp.
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