The narrative voice in the story is actually a relatively interesting facet of the novel. As the previous post suggests, it is an "adult" Scout, so the vocabulary and style of the prose is more adult-ish, but she also maintains some of the feeling of the younger Scout and in so doing provides a sense of innocence. At the same time that the childlike qualities of Scout are well portrayed, the adult understanding of past events provides for deeper insight and further examination of the various things that took place in her past.
An older Scout (This is Scout talking about 30 years in the future). Think about it like a journal, and this is Scout looking back at the events from her childhood. So, Scout as an Adult