Is the thermometer on the floor?
I've noticed some stratification of temperature within my cellar (8 ft x 7 ft by 8 ft high; cooling unit located in one corner near the top). I decided to put an oscillating fan in the cellar to run at all times (as opposed to the cooling unit fan that only funs some of the time). Even with that, my independent thermometer, which is located about 6 ft above the floor at the opposite corner from the cooling unit, reads one or two degrees lower than the sensor of the unit (located at the upper corner thereof).
I've adjusted the set temperature of the cooling unit to give a mid-50's temperature on the independent theremometer.