The Digital Elevation Model used to determine elevation does not go above 60 degrees N and below 50 degrees South, so much of Norway, Sweden and Finland is not covered. If you are above 60 degrees N it will return zeroes, even on the altimeter models (it overlays GPS data over the data collected by the altimeter based on the DEM, which has no data above 60 degrees N). The ascent is based on a digital elevation model that is used to compute ascent based on the GPS points collected. It is total cumulative ascent and it is an estimate that is only as good as the GPS points collected (which can be 20m-30m off in any direction) and the model used, which can have bad data in it.
If your watch has a barometer, it will come up differently than the one which using GPS. Our first generation of Multisport watches has the built-in barometer which takes the the elevation data from DEM but the new TomTom Adventurer, the built-in barometer overwrites the DEM data.