Colloquial, Relaxed, Colorful
Since our narrator is a 12-year-old boy who gets a D in English, it makes sense that A Day No Pigs Would Die isn't going to be told in standard college-essay language. And it isn't. Rob's not trying to impress anyone with this story, and he writes in a way that's very close to the way he talks: a natural, down-to-earth, hardscrabble kind of language, full of colloquialisms and informalities.
Not only is Rob, um, not exactly a master at the rules of proper grammar and usage, but the language he's grown up hearing and speaking is full of unusual, colorful expressions. This makes for some really original, engaging, vivid ways of saying things. For instance, instead of just telling us he's surprised and taken aback by Aunt Matty's grammar lessons, he says this:
I just sat there, dumb as a post. I guessed I didn't have brains enough to dump sand out of a boot. If she'd asked me if'n I was Robert Peck, I don't guess I could of answered a good stout yes or no. (6.40)
This does more than tell us what Rob is feeling at the moment in question—it tells us something about who Rob is and how he thinks. It's chock full of sensory detail and specific, concrete imagery that seems to spring naturally from his experiences living on a farm, working with his hands, and observing the people around him.
The book is full of examples of this kind of expression, like saying about the distressed Mr. Hillman, busy digging up the coffin of his dead baby daughter, that "his voice was an illness" (8.31), or saying that someone's opinion is "as wrong as sin on Sunday" (1.26). (Up for a fun game? Try to figure out where all these expressions might have come from.)
So while Rob's style may not win him any awards from his English teacher, it's just the right fit for this little story of a boy and his pig.