Inkbird ITC-308 WIFI control, accuracy etc

I have 3 of these, all controlling soil heating cables in a horticultural setting.

However I do not seem to be able to get accurate contol of the heat. The SV is set to 20 deg C but the overall swing of the temperatures is plus and minus 2 deg C (4 deg C total). This only being 10% control of the temperature.

Any ideas or thoughts please.

The ITC-308-WIFI is an on/off controller, it will turn the heater off at the target temperature, but if the heater environment used is slow to dissipate heat, or if the heater is more powerful, its residual heat may raise the temperature a little, which is normal.
Perhaps you can adjust the setting value depending on how it performs.

It seems I am having to leave it running for at least a day, for the process value to stabilise, i am now getting plus and minus 1.5 deg C (3 deg C total)