Originally Posted by
Brew1
Mine will do this on occasion. Don't know for sure but it could calculate elevation based on barometric pressure so if the barometric pressure changes during your ride it would give you a different reading when you arrive home. But I don't know for sure.
This.
Load your rides into Garmin Connect and it will 'fix' this based on the know surveys of the area - basically the maps know how tall a place is so it recalibrates your ride for you.