How did the Canterville ghost come to understand after his third attempt that he had been tricked, foiled and outwitted by the twins?

The Canterville ghost devises an elaborate scheme for frightening the Otises on his third attempt. He especially wants to teach the twins a lesson and decides he will sit on their chests, stand between their beds in the form of an icy green corpse, then crawl around the room as as a skeleton with one rolling eyeball. He is en route to enact his plan, which also involves scaring the senior Otises and Washington, the son who keeps scrubbing out his bloodstain, when he is met in the corridor by a ghost. He is so frightened he runs away. At the crack of dawn he returns, hoping perhaps to join forces against the Otises with this ghost. When he seizes its arms and its head falls off, he discovers the ghost is fake, made of a bed-curtain, sweeping brush, cleaver and turnip. It is at this moment, at dawn, that he realizes the twins have tricked and outwitted him.