Poorly written sections often correspond with sections where the storyline doesn't quite work, or the section doesn't really tie in well with the plot, or the author seems to just be riffing on some idea that doesn't deserve so much space.
The flip side of that is totally-unrelated sections that are way, way better written than the rest of the book, because they're about something the author is passionate about. (Oh man, if I was English-teacher-style forcing that sentence not to end in a preposition, it would have been really bad!) Like, where the book's historical fiction, and it's kind of plodding, but all of a sudden there's this large, eloquent narrative interlude about the comparative merits of knitting over crocheting. You know? Where you can just tell that this is what the author is really into.
Oversimplified Moral: Love what you're writing, and make sure it makes sense.