Temperature calibration - TRV showing -7° difference with external sensor

[pieri70]

Hello
I have a TRV (Sonoff TRVZB) that support offset.
My radiator is under a window and my wife opens completely the window for 2/3min each morning, so temperature drops a lot.
In BT I set the external temperature/humidity sensor, that is not distant from the TRV but it's not under the cold coming from window opening.
Then in BT I set the Calibration Type as Offset Based and Calibration Mode as normal
When i look at temperatures, after some time, the temperature by external sensor is 7°C higher than the temperature of my TRV (the offset of TRV is set to -7°C), this happens also in the evening when the window is closed by a long time.
Setting Calibration Type as Offset Based and Calibration Mode as None, then the two sensors measure almost the same value.

Is this normal??
How does calibration works?
Thanks everybody