Looking at the satellite map on gogle, the turns appear to coincide with distinct features. 6 wood loop the Horsham airport, 5 loops the Pensauken landfill & Haddon neighborhood with dintict highway features. 4 loops inside Interstate 476 along the wooded greenbelt. All good visual reference for the jet pilots. The crossover point in Philadelphia appears to be near the St Christopher's Hospital for Children and nearby Temple University Hospital.
Would summise they selected a specific cross over point and determined good reference points from there. Would be intersting to see what hospitals lie near the flight path.
Turn 4 to North is Bryn Mawr Hospital. Along the flight path between 5 and 6 are seven hospitals.
Didn't look at the Trenton area.