Ok, how to calibrate the humidity sensor?

I bought three Plus H&T. The original humidity values were off the charts too high, now with the current firmware they seem to be more realistic. But still they look like guesswork and are higher than compared to the measurements of this other, cheaper device. The photo shows a Plus H&T with the current firmware. The device on the left is a fairly cheap device that isn't calibrated, either. So which of these devices do I trust? And what do I do to find the actual humidity value for my rooms? How to calibrate the Shelly?

Why does it need an offset? How am I supposed to determine the offset? Why doesn't the device come properly calibrated from the manufacturer?

Also, is a one time calibration enough or will I have to recheck the values and then recalibrate the device on a regular basis?