GPS/Glonass: displays 50m too high?

I noticed a height difference of about 50 meters, when I move to points of known height (absolute altitude above sea level) in Germany, even when the accuracy is high (a few meters) and the horizontal position (i. E. in a maps app) is given very precisely. Jolla's value is higher than the real one. Android Apps and "GPSInfo" from warehouse give identical values, so I think it's the system. Someone able to confirm? Michael