Meggie's our main character, and in chapters devoted to her perspective, we only see things from her viewpoint (but always in the third person, because that's how this book rolls). So for example when Meggie first meets Elinor, they don't get along. Which is putting it mildly:
Once again, she looked Meggie up and down as if she were being asked to admit a dangerous animal to her house.
Meggie felt her anger make the blood rise to her face. She wanted to go home, or get back in the camper van and go somewhere else, anywhere, so long as she didn't have to stay with this horrible woman whose cold pebble eyes were boring holes in her face. (4.32-33)
The narrator is firmly aligned with Meggie here. We get Meggie's feelings and assessment of Elinor, but nothing that gives us a glimpse into Elinor's experience of this moment. So though the narrator seems pretty clued into the inner world of Meggie (she never says out loud she wants to go home, after all), they don't seem to have the same access to Elinor.
Here's the thing, though: the narrator's allegiance shifts. Though they hang close to Meggie for most of the chapters, they visit other characters throughout the book too, so we also get insights into the perspectives of other characters from time to time. Because of this, we know that Elinor is not, in fact, a truly horrible person and, implicitly, that Meggie's assessment of things isn't always to be trusted. By shifting perspectives, the narrator gives us a more accurate picture of each character.