100 feet higher than you started? Then it's going to rain.

I think others have addressed your problem already, but the accuracy of your barometric altimeter will be better in stable weather and worse when there's a front moving through. As others have noted, if you want the elevation data from gps waypoints, you can toggle that option in Garmin Connect

Now, on to the question that you haven't asked. You will soon find that the Garmin Connect site basically sucks. The uploads take forever and the data manipulation is terrible. To get much more out of your gps data, export your ride as a tcx file and then upload it to RideWithGPS.com. Alternatively, you can now upload the data directly from your Garmin. Much more satisfying.