ok, Mt Hamilton route was created by finding segment, finding someone that started near where I was going to start, and then copying their route to my Strava Routes.
If I create one by hand - this one took about 2 minutes - no elevation error:
Maybe the elevation error was introduced by the bike computer that captured the original route? And compounded by Strava not attempting to fix GPS data? I randomly picked a route to copy, have no idea when it was created.