When you turn on your Garmin it will acquire a GPS until it's good to go but that doesn't mean it is totally centered. If you wait 15 or 20 min you will get a better altitude reading. Due to basic triangulation problems, altitude is never very accurate in a GPS like a Garmin which is why most models have a function that allows you to set a known altitude for a given GPS location. If you set that known location you will get a better reading, but even with that set properly and with your Garmin knowing the set altitude of your starting point, when you return after a 22mi ride you shouldn't be surprised to learn that you gained and lost a few hundred feet over a perfectly flat course and upon returning you are 200 feet higher than when you started. There is nothing you can do about this deficiency. It's why surveyors (in part) don't use mass market GPS devices. They simply are not that accurate with regard to altitude. I'll bet if you look at your data you'll find your altitude changed when you got back home too. In your manual, find if there is a way to set a known altitude for your home. You can probably find that information, reasonably accurately, on Google Earth. Short of that, turn on your Garmin well ahead of leaving to let it center as best it can. Do it with your heart rate strap attached if you use one so that it "sees" the HR monitor right off the bat or you may have to turn it off/on for it to find the HR strap and then it has to acquire satellites all over again.