Can't vouch for that hill, but I concur you can find roads that steep. They aren't common, but they exist.

You can't necessarily trust the instanteous readout on your GPS--consider if one reading was off by -30m and the next reading off by +30m. However, the average will be close. The Garmin software does pull out the outlyers and correct against topo data (if you send it up their website).

The organizer of one of my favorite charity rides likes to describe the SW corner of mystate as "a place where they'll hang asphalt off the side of a cliff and call it a road". I suspect that a lot of rural areas in the country that haven't seen a lot of citification are the same.