 The devil doesn't know everything, and he only knows what God reveals, and God at this point of the story had not yet revealed how he was going to send the Messiah. God had said he was going to send the Messiah through Abraham's line, and Abraham produced Isaac, and Isaac produced Jacob, but now for the first time in the story, the devil has a problem because Jacob had 12 sons, which is the one through whom the Messiah is going to come.