First Clutch on the Ground

I am seeing the same thing. My incubator ambient is reading 90 and my box interior is showing 88. Just remember, the thermostat's sole purpose is to maintain a steady/constant temperature.

Imagine if it isn't calibrated correctly, you would set it for 89, it would display 89, it would maintain that 89, but in actuality the temperature could be 92. Again, this may be an exageration, but you get the concept. As long as the thermostat is maintaining a constant temperature, you need only concern yourself with the reading of the thermometer and heat gun for the inside of the tub. Remember the heat gun is the bible and the thermometer should corroberate it or at least be off by the same margin repeatedly.

If it is running a little low, say 86 or 87, then bump up the thermostat until it comes up to the desired temperature in the interior of the box. It shouldn't matter if the thermostat is reading 91, or the incubator ambient is 91. What does matter is the temp inside the box which you are using the thermometer and temp gun to record and compare. The thermostat will regulate the ambient temperature, which in turn will regulate the interior temperature. I hope that make sense.