One thing that jumps out at me from this map is the low number in Cook County, IL (Chicago land). I would have expected more given it is one of the hotbeds of gang members as well as the stomping ground of Obama and many associates.
I doubt much of the gang-bangin' there is handled in federal courts. The difference could be one of 'filing style" by the prosecutor, or by style of the magistrates. I am sure there are significant differences from one big city to another, in the style or formalities of work. Some magistrates may seal as a matter of routine, everything! Others may, as a matter of routine, seal NOTHING unless explicitly asked, in the right way.