I generally keep track of my rides using a cycling app. I have noticed that the reporting of the elevation gain statistic by the apps is wildly variable. For example, here's what the app told me for the last 10 times I did my frequent 18-mile ride:

Elevation gain (in feet):
866
1287
1476
1297
1296
1141
1460
1134
1251
1224

These are the same ride along the same path on different days. These were tracked with the Samsung Health app in my Android phone, but I've gotten similar inconsistency with other apps (including Strava and Map My Ride) and other phones (including an iPhone.) How do the apps keep track of elevation? Do they use GPS data (which I think would be very accurate) or barometric pressure (probably less accurate)? Has anyone else noticed anything like this?