I had the same question when I was setting up my controller and freezer. You are measuring the ambient air inside the freezer and that is not going to give you the temperature of your wort. Tape the probe of your controller to the outside of your carboy/bucket under a piece of Styrofoam or other insulation, and you will get pretty close to the temperature of your fermenting wort (as long as you have liquid in the fermenter). Don't try to get it set without having liquid in fermenter. I set mine up with 5 gallons of water in a carboy. My set temp for the test was 65F and the temp differential was 1 degree. The compressor would kick on when the water got to 66F and shut back off at 65F. The coolest the water got, even with the ambient air in the freezer around 57F was 64.5F. Your particular system will not be exactly the same as mine due to different freezer sizes, volume of wort you are fermenting, etc., but the only way to get a true reading is either taping the sensor to the carboy or using a bung that has a hole for a thermowell as well as an airlock, and placing the probe in the thermowell.
You will drive yourself crazy going by the ambient temp in the freezer.