Although I can only speak from personal experience, I'm sure my case is the prototypical case for all classes. When a professor assigns reading, they don't actually assign reading. Sure they give you a number of pages, and if they're feeling generous, they'll actually specify which sections to read, but for the most part, we just get page numbers. A textbook isn't just filled with text, it comes with a bunch of other material from side bars, mini boxes, photos, statistics, graphs, captions, and sample cases. Although it's never explicitly stated that you have to read those, it's implied.
I call that extra reading, "bleed reading." Bleed simply refers to the edges of the page in the printing business. You have to make sure you have a large margin or else the printing machines will cut off parts of your layout. All these tacked-on extras thrive on the margins, away from the meat of the textbook. That's the reason I call it bleed reading.
Common sense dictates that you read it because you know you're going to have to read it anyways. Besides, you never know, you'll probably learn more from bleed reading than from regular reading.
