Regional
As narrators, both Tilly and Howard are grounded in their particular place and time. Tilly's voice is distinctly from the rural Midwest, her dialect obvious in the way the author transcribes her speech. For instance:
That's the way we done in them days. You was sewed into your underwear in October and didn't see yourself again till late spring. We thought if we got nekkid and washed ourselves in the wintertime, we'd catch a chill that would carry us off. (2.11)
Similarly, Delphine's speech indicates she is from New Orleans. It's sprinkled with French words and syntax that indicate English is her second language, like so:
"Eh bien," she said with a little shrug. "Well, me, I would like to examine the town. Are there shops?" (4.66)
Place matters in this book (more on why over in "Setting"), so it's fitting that the writing style drives this home for readers.