Simple, Direct
Maybe it's because our viewpoint characters are young folks, but their way of narrating what happens to them isn't very complicated or fancy. Sadima, for example, notices the things around her and describes them in plain language:
Sadima woke before daylight, as she had all her life. But there was no sound of roosters, or goats bleating, no sleepy owl's homeward cry, and it took her a moment to realize where she was. (19.13)
Simple, clear, and unambiguous; we don't have to puzzle through polysyllabic words or wonder if she's hallucinating.
Even with Hahp's viewpoint, which does feature some freaky magical stuff that makes him question reality, we still see the same clarity and directness of style. Like when old-Franklin offers to help Somiss demonstrate something for the boys, Somiss just tells Franklin to leave:
Franklin looked sad, disappointed. For a few seconds they just stared at each other. Then Franklin simply walked away. No disappearing, no opening in the rock wall, no magic. He left the way we had come in. (20.14)
In addition to inspiring our sympathy (aw, poor Franklin), this quote shows just how uncomplicated the language the story's told in is.