You have to remember that the “height” of the CesiumGeoreference refers to the height above the WGS84 ellipsoid model. This is often unequal to the actual height of the terrain above mean sea level.
If you want the height above the terrain, you’ll have to use Physics.Raycast. If you want the height above mean sea level, see Kevin’s post on this thread: The height of the sea is not 0 - #8 by Kevin_Ring