Zip code price differentials reflect the underlying "discounts" of the
PPO network(s). If the MCO is able to negotiate more favorable discounts from providers in 30327 than they are providers in neighboring 30328 the rates will reflect those differentials.
Other factors that come in to play include proximity to trauma centers and the respective levels of those trauma centers, utilization, social demographics.
Typically, high income zips have greater utilization and higher claim levels than low income zips.
Of course getting an actuary to admit that is like trying to learn the secret formula for Coca-Cola.