A communication from Garmin on 9 May 01:

"The normalized pressure is only valid immediately after a calibration. In order for us to accurately compute it after a calibration, we would need to have an independent source of elevation. What the unit does subsequent to the calibration is to continue to assume that the unit is remaining stationary at the last calibration elevation, and that any ambient pressure variation that is measured is due to atmospheric pressure change, which causes normalized pressure to change as well.

You should see the altimeter functioning very accurately after you calibrate it, and that is our priority in this product. Normalized pressure was provided so it could be used to allow a user with accurate knowledge of his elevation to compute a normalized or sea level pressure. The normalized pressure that is displayed is NOT the
pressure that is used in the standard atmosphere model to compute elevation. There is no loss of accuracy in the altimeter portion of the unit just because the normalized pressure we display is incorrect. Remember, normalized pressure is based on the last calibration elevation.

Let us point out that there is not enough information for the sensor to be an altimeter and a barometer simultaneously. The unit does not use GPS elevation to try to update normalized pressure values because
that answer would just be wrong as well."