It is because drawn has three meanings, and they are (1) drawn with a pencil, (2) exhausted, and (3) pulled across a window, and meaning 1 refers to both the face and the curtains, meaning 2 refers only to the face, and meaning 3 refers only to the curtains.  If I try to say the joke to myself, making the word mean the three different things at the same time, it is .  like three people trying to talk to you at the same time about different things.  And that is why there are no jokes in this book."