This has to do with hysteresis.

Hysteresis at a heater is like a buffer zone around the set temperature.

Suppose you set the heater to 22°C, it will not immediately turn off when it gets exactly 22°C. Instead, the heater will stop heating when it gets slightly above 24°C and start heating again when it gets slightly below 20°C.

This buffer zone of 4°C - hysteresis - prevents the heater from continuously switching on and off when the room temperature approaches the set temperature. Not only is this switching seen as disturbing to the user, the hysteresis also helps with energy savings. The heater thus operates more efficiently and the room remains comfortable without unnecessary fluctuations.