Do the professors want us to expand our learning by doing more? Because I’ve been there before when I saw something on an exam and we never learned it at all. Maybe they want us to do more research on our own instead of just teaching us everything, to apply our learning, despite what’s on the study guide?
Lots of possible explanations-- they mentioned it when you were out of the room, or thought they mentioned it, or it was in the notes and you missed it, or it’s phrased differently than what you thought it meant…
The study guide is meant to be just a guide, not a promise.
And teaching you to think, to apply your learning is probably the best reason of all.
These are questions for the professor. No one here can tell you what’s in his/her mind…
In many classes, the lectures and discussions cover different material from the textbook.
One possibility is to separate the best students (who go extra miles and will get A’s) from the ordinary ones.
That’s one great reason to do the assigned reading.