1. The set point value of the temperature is the number that triggers the unit cools to, meaning the compressor will turn off when that set point is reached.
With that said, the fan near the already cold evaporator continues to move air across the evaporator for several minutes after the compressor stops. So the interior of the cabinet - particularly on the smaller units - will continue to drop, 1°F or possibly 2°F degrees.
2. The trigger that tells the compressor to kick on is set in the internal programming, it requires a 5°F degree variation from the set point before it begins to cool. So, in your example, where the upper compartment is set to 55°F, the compressor will not turn on until the upper compartment is 60°F degrees. You will note, that is in line with what you are describing.
3. The sensors which provide the temperature reading are measuring AIR temperature. Please be aware, AIR temperature fluctuates much easier and quicker than LIQUID temperature, therefore even though you are seeing 4 to 6°F degree temperature swings on your display, your wine is not experiencing such dramatic changes. Generally speaking, with a 5°F degree air temperature change, the liquid change is probably about 1/2 to 1°F degree.
With all that said, if your unit is operating within normal parameters as described above, it is not in need of service. If you have any further questions or require additional assistance, please do not hesitate to reach back out to us.