The student who already knows the answer may still need to learn how another person reaches it.

Knowing the answer and understanding how someone else gets there are different achievements. The student who is ahead still has something to learn in watching a slower route, both about the subject and about explaining it, which is the harder skill of the two. Being finished with the question is not being finished with the lesson.

If you already know something being explained, listen for the route rather than the destination. Try teaching it to someone struggling. Almost everyone discovers their own understanding is thinner than they thought at the point where they have to reconstruct it.