I would test your Thermo Pro 1ST: Check both probes side by side in the pit*. The reading should be the same or very close. If not switch the probes in the T P to see if the difference is in the probe or the T P.
As Bent stated, takes ~ 45 Min for the pit to stabilize. My Pro's temps are within a few degrees of each other, ONCE STABILIZED. The pit must
"heat soak". Air heats quickly vs the mass of the pit.
Let us know how you make out please.
* Could ice-bath both probes and T P circuits as well.