The altitude is measured via the pressure sensor. The barometric pressure depends on the altitude. The AlpinerX processor measure the barometric pressure and computes the actual altitude.
The pressure also depends on the weather conditions, this is why the altitude can change with the weather conditions. For this reason, the altitude must be calibrated from time to time.
In the AlpinerX application, in the "my watch" page, "calibration" menu, you can calibrate the altitude. If the GPS of the phone is ON and gets a good altitude position, it will propose a value. Enter the actual altitude of your position to calibrate the watch altitude.
In the menu "general", you can turn ON "auto altitude calibration". The phone will regularly read the phone GPS altitude and send it to the watch as a calibration value.