Altimeter inaccuracy

I did a run today, which I run very often, and the altitude meters were fine. I compared them with Strava's autocorrect and everything is fine. I didn't change any settings. I wonder if the satellites were at fault...

Martin, altitude meters are measured from barometric pressure, not from gps. I have many times on 5 km run from the same start 10-15m differences. I do calibration before every activity...

Did you start and finish at the same place and always in the same way?!

Yes, start and finish is always form the same place, from my home. Eight out of 10 times there are differences in altiutude gains, ets 520 +/ 510 - from the same place

I think it's a hardware problem because I run with other people who have the 955 and their heights are perfect. this afternoon I will run with my friend and post the two graphs 

I have the altimeter sensor mode set to "Automatic". In the shooting mode options, the option "Automatic", "Altimeter only" or "Barometer only" Why do you think that altimeters are only measured with a barometer? Below is a picture from my run today. the run was out and back and the altitude meters of the start and end run are identical.

Its measured always with barometer, by other brands too. 

A barometer watch continuously measures absolute air pressure using the built-in pressure sensor. Based on this measurement and reference values, it calculates altitude (altimeter, or alti) or sea level air pressure (barometer, or baro).

There are three alti-baro profiles available: altimeter, barometer, and automatic. The watch uses the automatic profile by default for interpreting air pressure changes as changes in altitude or weather based on your movement.

Look at your ascended and descended height meters if you have the same numbers after each activity

Btw im from Slovakia too :-)

The difference between the total ascent and descent is a maximum of 6m, which I observed in about 15 of my runs. But I am a special case, because I live in a block of flats with two entrances and they are not at the same altitude. But I will watch it.

Jj, všimol som si že si zo Slovenska podľa obrázkov z Tatier ;-)

I usually follow loops rather than out-and-back. But I do start and end at the same place. When I look back at recent walks, it says I descended about 20-30 feet more than I ascended in an hour, with a total ascent around 230 feet.

Looking closely, the plots show sensible ascents and descents based on the terrain I followed, but the very end may be slightly truncated. So some of this ending error may be that I press END just a few steps after climbing a slope to my start/end landmark. I guess I should try to delay ending the recording next time, to see if it will recognize more of my final climb and reduce the ascent/descent mismatch.

But there could also be a general trend through the whole hour, where the apparent elevation is decreasing slightly. It would make sense to me that the air pressure is increasing through my evening walking time. The same cooling that makes it more comfortable would also mean denser air moving in.

As I understand it, typical diurnal variation could have a pressure change at the same magnitude as 15-20 meters of elevation change, all within a 6 hour period. There are peaks in late morning and late evening, and troughs in early morning and late afternoon.