Heat tape is not going to be consistent temp across the whole tape. That is the only downfall to it. You need to just temp gun the inside floor of the cage/tub all around where the heat tape is and just set up a happy median. It is going to be a little hotter in some spots and cooler in others. In all my cages, the flexwatt varies from 84-90F. The snakes will find a spot they like on it and sit there. And then move off when they are warm enough.

You can also move the probe to the hottest spot and set the temp from there if you want.

And to answer your question, are you taking the reading from the exact same spot as the probe at a distance of less than 1 inch? Try temp gunning the probe itself right on top of it. Also, maybe the temp gun is off. Try putting a temp gauge probe on the same spot next to the Herpstat probe. See if they are close.