The answer to the question can be found on page 163 where Otis comes out and testifies. The reason why everyone is shocked is because they all thought that Otis was just some simpleton who worked at the soup kitchen, and they may have overestimated him. They did not know he was a hired private investigator who was hired precisely to find out the truth of the heirs. That came to them as a huge surprise.
you can find the answer on page 163 of your book.