Well, if it's not touching the substrate, then It's measuring the air above it and not the substrate wich is what your snake will lie on to "bask".

Try putting the probe on the substrate for a few hours, just to see what tempereture the substrate is at. This is important for snakes lik BP's because they lie on the substrate as opposed to climbing up branches, so the heat of the substrate is what they'll be getting most of!