I don't think there is anything wrong here. It would have helped to know where exactly you were parked at the airport (KAPA). The latitude and longitude of the location of the airport is where the distance is measured from. It is quite possible to be parked 0.5 miles away from that point. Airports are rather large If you want to verify this, in the X-Plane menus go to Location -> Local Map. Then click on the airport symbol on the map and in the upper right of your screen there will be data displayed, including the distance you are from that point. This should match the Corvallis map display as well as the Nearest Airport data on the Corvallis PFD. This is NOT a unique problem with the Corvalis. I checked the Corvallis at KAPA and it matched X-Plane's data exactly. I can't speak for other payer airplanes, but the all the stock aircraft should match the Corvalis. If you have a specific example with details, I will check it out.