Scout, the narrator, is an avid reader, and she narrates like one. Harper Lee achieves this effect stylistically through the frequent use of figurative speech and complex vocabulary. Scout's more elevated manner of speaking and writing contrasts with the various dialects used around her.
Another note: Scout will often use words in contexts that contradict their typical usage, at times to ironic effect. This is a stylistic choice often made to draw out the idiosyncrasies of characters or situations. For instance:
I contented myself with asking Jem if he’d lost his mind.
In this sentence, the word “contented” contrasts with the statement that Jem has “lost his mind”: one does not typically associate contentedness with instability. In addition, the relatively lofty turn of phrase ("contented myself") contrasts with the more everyday register of "lost his mind." These two sentiments and levels of diction contradict one another; yet, in their contradiction, they elevate and deepen the sentence’s meaning. Scout “contents” herself with asking Jem about his mental health. The use of this word implies that Scout does not consider her brother’s mental state a serious issue—that asking him if he has “lost his mind” is simply a way to emphasize her own feelings. The question is not being asked in earnest, but rather directed against Jem as a kind of conversational barb Scout is clearly “contented” with and used to making.