I'm guessing it's the mass of the beer causing the problem for a couple of reasons: First, simple thermodynamics. You're cooling a large mass of liquid using air. By the time the liquid drops to the temperature you desire, you've way undershot the actual temperature needed to hold it there (assumption, the probe is in contact with the beer). Or you've cooled the air to the point your controller kicks the freezer off but your beer is not cooled to the proper temps (assuming you're measuring the temperature of the air in the freezer or, as you mention, a smaller volume of liquid). I use the Inkbird controller in my fermenting refrigerator with an incandescent light to balance with the probe in the beer. By the time the beer has stopped producing its own heat due to fermentation, all the yo-yoing of the temperature has stopped and everything is near equilibrium. By the way, use the Inkbird in celsius rather than Fahrenheit mode - it's more accurate and the temperature range is closer.