My impression is that a graded road with bridges is not the sort of elevation change that you would see on a contour map that the 'public mapping data' might be using. It's funny to look at the elevation profile from strava for the local rail trail vs what comes out of the garmin's altimeter. Strava looks like a saw tooth, which isn't correct if you can imagine how a railway grade really looks. heh

In other words, elevation changes are smoothed out when they build roads, and if the public data isn't based on how the road was built, then it will instead show lots of ups/downs that don't really exist.