Just to go back over a few things op has stated. Air flow cools damp substrates at a much faster rate then dry substrate. So the fact that the snakes urine comes out at a snakes body temperature is moot, because that alone can cause a wet thermostat probe to read a lower temperature then the surrounding area. Second you said a snake laying over a thermostat probe has the same insulation of substrate as one mounted between the tank and uth is also incorrect. A snake with any substantial mass brings its body temptature with it.
An 80 degree snake laying on top of thermostat probe set for 90 is going throw off incorrectly regulated temperatures untill the the snakes temperature reaches 90 degrees. With the probe under the tank it touches the uth directly. Meaning that even if your snake is 80 degrees lying right on the glass above a mat set for 90, the temperature of your snake will rise at a slower and steadier rate never ever reaching above 90.
