Amazfit Watch faces

My amazfit stratos shows different altimeter when I'm on the go than the altimeter recording on gpx file it exports after. I know the right altimeter in some points, so I notice that the watch is screening the right altimeter most of the times, but when I see the same points in the exported gpx file, after the activity, I see differences, especially in the beginning and in highest points.
Does anybody know why? And how can we fix this?

My understanding is the altimeter is based on barometric pressure, and pressure changes. So changes in pressure result in altimeter reading changes. Pressure can change not only with altitude but weather conditions. I have noticed this on two different hikes. One ascent to 1750 feet and another to 3500 feet. AFIAK there is no way to calibrate it either.

My review that shows this here.
https://youtu.be/CxJih7wBm-c