The best way to set it up is to have the tstat probe outside the cage, stuck on top of the uth under the cage. That way it can not be moved by the snake causing your tstat to run away and burn your snake.

You can still keep the thermometer probe inside to monitor the hotspot because it does not matter if it gets moved.

Either or even both devices could be a little off. I had a hydrpfarm once and it never seemed to Agree with my thermometers. It also died after a few months. I was lucky when it started going out and only sending half power to the uth and the display was flashing crazy. Watch it close.

My thought is how you attached the tstat probe. If the probe itself is covered it could be insulated and holding heat. You need to have the probe body uncovered. Place tape on the wire just behind the probe to avoid trapping heat at the probe.