Maintaining Heat

So, I just got a new ball python. Right now I have the zilla heat pad 1000 watt and zilla thermostat. I placed the probe kinda middle/side of my heat pad underneath the substrate. Its out of the way of the hide, but still on the heat pad enough that I think its getting an accurate reading.

My thermostat, however, says that the hot side is below 90 degrees, almost low 80s. My substrate is aspen/wood/mulch (that was the subrate I bought him with. Im going to switch it with ecoearth in the future). I've read that I should keep the probe below the substrate so in case he burrows down he wont burn himself, but should I be worried that my thermometer is reading so low? I've ordered a lazer thermometer so idk what the substrate is actually reading yet.