When flying, GPS altitude seems to be constantly shifted.
Solution:
Mobile GPS altitude can be shifted. My experience is that altitude is shift around 120 feets up.
After application start and get first GPS fix, have a look at GPS altitude. If it is shifted from correct altitude, you should adjust it. Press Menu / Alt correction and adjust it.
I adjust it usually about 120feet down. It you find that value of alttitude correction is about constant value, you can set Default altitude correction in Preferences.