Because he knows less

It often happens that two schoolboys can solve difficulties in their work for one another better than the master can. When you took the problem to a master, as we all remember, he was very likely to explain what you understood already, to add a great deal of information which you didn’t want, and say nothing at all about the thing that was puzzling you. I have watched this from both sides of the net; for when, as a teacher myself, I have tried to answer questions brought me by pupils, I have sometimes, after a minute, seen that expression settle down on their faces which assured me that they were suffering exactly the same frustration which I had suffered from my own teachers. The fellow-pupil can help more than the master because he
knows less. The difficulty we want him to explain is one he has recently met. The expert met it so long ago that he has forgotten. He sees the whole subject, by now, in such a different light that he cannot conceive what is really troubling the pupil; he sees a dozen other difficulties which ought to be troubling him but aren’t.

From the Introduction to Reflections on the Psalms, C. S. Lewis