Maddy and Az were cosmetic surgeons on the Committee for Morphological Standards—or to make it easier, the Pretty Committee. That's the group that decides for all the cities what sort of surgery should be acceptable for the pretties.
They were doing research on the pretty surgery and found that people had tiny lesions all over their brains. It turns out that lots of pretties are brain damaged—and brain damaged in the exact same way.
The only pretties who don't have lesions have jobs that require quick thinking: firefighters, wardens, doctors, Shmoop writers, Special Circumstances, etc.
Well, that explains why the rangers that Tally met were different than most of the pretties she met in the city.
In fact, it seems like the lesions are done to the pretties on purpose. Maybe that's why there's no war and everything is nice in the city—almost everyone is brain damaged to be happy and slow-thinking.
The operation changes the way people look and the way they think. Uh-oh. Not cool, future dystopian society.