DIAGNOSIS The mechanical thermostat is miscalibrated or the temperature probe on the back wall is being artificially cooled by direct airflow from a vent or fan.
FIX Use an independent thermometer to verify. Place a glass of water with a thermometer in the center of the middle shelf. Wait 24 hours. Note the actual temperature. If it's more than 10°F below the dial setting, the thermostat is faulty.
IF THAT DIDN'T WORK Relocate the unit. Ensure it is not placed directly under an AC vent or in the path of a ceiling fan. Cold air blowing on the external cabinet can trick the internal probe.
HEADS UP The thermostat dial is not a precise linear scale. The 'Min' setting is approximately 34°F and 'Max' is approximately 50°F. Settings in between are estimates. A dedicated external thermometer is required for precise control.