Temperature-controlled environment are usually driven by a thermostat, which has slightly different thresholds to turn heating/cooling elements on or off, to avoid switching this every second or so as soon as it goes above/below the desired temperature.
For instance, for a target temperature of -27°C, it could turn on cooling until it reaches -28°C, then turn it off and keep it off even if it goes back up a little, until it reaches -26°C, where it will turn it on again, and so on. This is called hysteresis.
Without that, it would cool until it reaches -27°C, stop, then start again when it goes up to -26.99°C, then stop again when it's back down to -27°C, and so on. This would happen a lot more often then it does with hysteresis. With complex mechanical systems like cooling or heating elements, this is usually not a very good idea.
The fact that it moves back up so quickly may be the sign that there is a (small) source of heat inside the room, or that it is not correctly isolated. Or it may be perfectly within specs.