Not a lawyer, but a couple of points:
1. It is common for the SCOTUS to take up cases that have had differing rulings in the lower courts. That is the case here.
2. Can the District of Columbia, not being a state, even have a "well regulated militia" ?
3. Saw a clip of the D.C. Mayor arguing that many of the homicides in the district were caused by handguns so they should continue to be banned. This argument would seem to indicate that the DC ban doesn't work anyway- a strang argument to make in favor of the ban.