Naomi has good advice, move the thermometers to where you think the snake will be, thats where your temps should be taken from and its unlikely to be part way up the wall for any length of time.
I would move both to the side walls, or just 'drop' one on the floor by the water bowl and test the temp there, but move it when the occupant moves in, and only turn the light on when you are home to look at them (the snakes) assuming they are in a room that would get some natural light during the day (NOT direct sun on the tank, they will overheat quickly) and watch the thermostat dial. they can easily be moved by a wandering snake... ,
as a side note, i hope you didnt stick the thermostats on with double sided tape, it has a habit of softening with heat and the thermostat coming away from the tape and theres a chance your mate could find themselves stuck to the wall .
Once you decide where you want the readings to be done from, silicon them in place.