To expose the illogic of the situation, consider what happens after the prisoner draws his conclusion. Then, on Saturday the hangman does arrive, and he is seriously surprised after all. It'll be a complete surprise. You'll never guess it. It's that bike you've always wanted.
Days pass and the boy's bafflement rises.
My father doesn't lie. The day comes and his father gives him the bicycle. So, you might say, the very reasoning that gave the prisoner hope condemned him in the end. A better way of looking at it is that what one person knows to be true, another does not. Well, you get the picture.