The narrator in Lord of the Flies moves back and forth omnisciently between different scenes and thoughts. Take Chapter Eight, for example, where in the space of a few pages we get Jack hunting, "happy and [wearing] the damp darkness of the forest like his old clothes" (8.181); and Simon watching the flies swarm "black and iridescent green" on the pig's head; and then Piggy "flush[ing] pinkly with pride" when he understands that Jack is accepting him (8.265). Three different characters; three different places on the island.
What's the point of all the omniscience? It lets us (and the narrator) stay objective. We might have a slight bias toward Ralph, but in general we just see things happening without the filter of a particular character's judgment. We can be right inside the events, seeing Jack as a terrifying, painted chief; or we can be way up high and objective, seeing Jack as a stupid-looking little boy in a crazy black hat.