I can't say I understand it. From what I know, the two have a messy history.

Zaun sounds like if Crime Alley was a whole city instead of just a neighborhood. Living in a place like that means you learn to always work in the gray areas and on the edge. You're used to the rich folks looking down at you so you harden. You scrounge up whatever resources you can and claw your way up from the bottom. A whole city of people who have had to live like that means you get some pretty strong bonds against outsiders, even if the internal politics are messy as hell.