The OnMove 500 gives you the instant altitude, as well as the cumulative elevation (positive and negative) thanks to the GPS technology.
When you transfer your data to the app, the system recalculates the real altitude based on your route and on the map. It is therefore possible to observe some variation between the data on your watch and on your smartphone or computer.
Measurement via the GPS signal is not as reliable as with a built-in barometer.
When there are many small changes in altitude, the error will be more important than with important changes in altitude when climbing a mountain