You might have noticed that there is a difference in the distance/elevation data while you were recording your activity vs. once your activity is saved and the data has synced with our servers. The reason is that GPS data from your phone is notoriously noisy. We are able to do moderate filtering within the app during the recording of your activity to keep your data somewhat clean, but a good chunk of data smoothing and cleanup occurs during sync.
Once your activity is complete and you choose to save your file, it will automatically be queued up for syncing the next time you have a stable data connection. During sync, our web servers are able to smooth out the data and recompute your stats to be much more accurate.
In regards to elevation data changes specifically, the elevation gain will be most accurate once you have actually saved your activity.
Once you save your activity, your map is uploaded to the server, at which point we discard all the lat/long points taken from the locations while you were recording and use our own system to replace those elevation values. We then recalculate the total elevation gain based on those values which will most often result in a lower number for total elevation gain then while you were recording your activity.