So I researched this a bit and here is the answer:

The grouping of devices origins from this commit:
https://github.com/domoticz/domoticz/pull/847

and the logic is:
https://github.com/domoticz/domoticz/issues/276

Unfortunately it doesn´t work very well with other sensor combinations that Temp/hum/pressure - like mine with voltage, impedance and so on..

I found a workaround by naming all sensors v_hum - then they are placed in different groups and are possible to work with in domoticz...