Barometric altitude is different than geographic elevation. One references air pressure and the other is absolute height referenced to a geographic reference. If you want them to be the same you need to correct for changes in barometric pressure in much the same way as airplane altimeters are corrected. Since your unit doesn't have an adjustment built in you can find the current barometric pressure either from the weather service or flight service, convert to elevation and then record it as your starting waypoint altitude. They may even do the conversion for you>