This is voter management, making sure the Democrats win by about 60% uniformly. There's no benefit to winning 100% of the inner city while only 45% of the conservative suburbs. They also need to avoid excessive genetic and cultural inbreeding and mix things up periodically. Without that after a few generations they would be producing essentially zoo animals.
Having spent a fair amount of time in Antioch, I’ll bet a shiny nickel that this is exactly right.