Originally Posted by mschwett

both devices have a barometric altimeter and GPS. what apps/software are you using to view the results? many map based systems (e.g. strava) will correct the device altitude readings based on map data. said map data varies in quality greatly, so you have three different potential altitude sources:

1) the barometric altimeter
2) GPS altitude - essentially useless
3) elevation from maps by correlating GPS location

different software will absolutely use different methods to reconcile the variation between these three sources.