Okay, here's how it works. Your thermostat is set to hold a temperature. But it can only turn your heat pad on or off. So when the heat pad gets too hot, the thermostat turns it off. When the heat pad gets too cold the thermostat turns it on.
What I do is basically ignore the thermostat. I have a thermometer with a probe on it and that's what I use to monitor the temperature. I keep thermometer reading between 85 and 87. To do that I actually set my thermostat more like 95. In my case the thermostat probe is on the outside of the pad, outside my snake's enclosure, but the principle is the same if the thermostat probe is inside next to the thermometer probe. I just set the thermostat to keep whatever temperature I want as measured but the thermometer inside on the glass under the aspen in the inside of the warm hide. I keep it a little on the warm side of 85 because it is under the aspen unless my snake burrows down to the glass. 87 is still safe if my snake burrows.
I know this sounds complicated but it is easy when you have your thermometer in place and monitor the temperature using it.