I guess there's different ways of computing elevation as well. If you do a loop from sea-level up a 15,000ft mountain and back, your destination is the same as your starting-point and the net elevation change is zero.
G Map shows the elevation changes for the whole route, not just start to finish net change.
Here is the message that it is giving me at the moment. It appears as if there is a problem with their data source.
Elevation only available on routes with two or more points, and in the U.S. You may also see this message when there is a problem with the remote service that provides the elevation data.