The Ashburn North garage is of course touted as being part of the new station, but I believe it was built by the developer of the large mixed use project that is going in north of the station. Right now there are still large surface lots but eventually those will be built on so they needed a large garage for people coming to the mixed-use center as well as for Metro riders. And that's the reason they did not locate the garage right next to the station but instead closer to the development. As for the South entrance it does seem bizarre that they located the parking garage so far away from the entrance to the station. As a terminus, and one located in a county where you expect most people to arrive by car and not by bus, you would think they would have put the garage closer and that there would have been covered walkway between the garage and station.
Living in Broadlands, VA; Working at Half & L SE, DC.