I have a Garmin Oregon 550t, which is a very different unit, but probably has a few similarities under the covers. I've noticed that altitude is the least accurate metric the thing can give me. The lat and lon measurements seem to always be within 10 feet when I plot my data over a sat map, but I often descend as much as 300 feet at the beginning of my rides ... which is odd in that I live at the bottom of a hill.

So far as I know, the Garmin units have barometric and GPS altimeters. The barometer can get you to about 10 feet accuracy, in general, but goes wonky when the weather changes. On the other hand, the GPS is supposed to be accurate to about 150 feet, and that's what you get, regardless of weather. I think the best approach is to calibrate the barometer often; I know the elevation at my front door, and try to remember to check or set it at the beginning of each ride. I'm not very successful at that...

On the other hand, when I go kayaking, I ignore the altitude plot all together. The three to four foot swells are inside the margin of error, so the data is useless to me. If I were cycling in Florida, I'd probably be thinking along the same lines...