I think it's an artifact of when the District was shaped like a perfect diamond, 10 miles on a side, with the land split between Maryland and Virginia. After the Civil War, there was a sense that the Federal government would never need that much land, so they gave Virginia's portion back to the state. That land became Arlington County, and includes part of the city of Alexandria. If you look at a map, those two geographic entities "complete" the diamond.
JoeR
My understanding is that the federal government was so inept in managing the affairs of the city and county of Alexandria that the locals asked to be returned to VA. The retrocession happened in 1847, so it pre-dated the civil war by a couple of years.