Both your thermostat and heat gun are correct.
The issue is that the heat of the UTH needs to penetrate the bottom of the tank and the substrate to reach the surface, where you are measuring the temp.
The substrate is insulating the heat mat. So, naturally you need higher temps to penetrate through an inch of substrate. If you were to dig away layers of substrate and take temp readings, you'd notice that as you go deeper into the substrate it'd get warmer and warmer.
I have a BP on a UTH right now. My thermostat reads 97F, but her surface temp is only 90F, as measured by the heat gun. So I lost 7F in heat on its transfer to the surface.
With ambient temps as low as yours, I'd probably opt for a CHE, or RHP. Its much easier to get a good temp gradient that way.
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