I also use a Combustion thermometer (grilling in my case). My understanding is that the ambient temp is not the same as my grill thermometer because the area right around what you are cooking (where fhe ambient temp sensor is) is cooler because of the moisture evaporating from the food you are cooking. Not sure if you tested just the thermometer in the oven without inserting into something.
More details explaining this from the creator of Combustion: https://t.co/2Y96BJHHOz