What dictates the lvl of zombies, will they change in a given area?

Zombies are random but some areas will only have low levels. Your state will have regions / bands that circle the map. The central areas are higher level and the outer areas are low level.

In the high level areas you can get low to high level zombies appear although higher levels are more common. On the very edge of the map you will only get low level zombies appearing (1-5).