Why do some zombies not die immediately on sunrise?
Sunrise does not directly kill zombies, it causes them to sustain a certain (fairly high) amount of damage per frame (the game runs at 30 frames per second). High-level zombies have many, many more hitpoints than low-level zombies. Thus, the sun kills low-level zombies nearly instantaneously, but high-level zombies take much longer. This is an intentional feature: the stronger zombies have gained resistance even to the sun.
