Freezer temperature is supposed to oscillate.  Here is an example of the temperature profile of a -20 freezer. 

Your freezer has a compressor that does the work of cooling the inside of the freezer down to its setpoint. Once the freezer detects it has reached the setpoint, it shuts the compressor off until it is needed again. This causes the temperature to oscillate within a manufacturer-specified range. (usually a few degrees).

If you see the behavior of your freezer change significantly, we would generally recommend you have a freezer technician look into it. 

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