Clare, the rich list are often “self reported”. The really smart people stay away from such fripperies as rich lists because they don’t want to be targeted and ambushed by charities, get rich quick schemers and the ever present clowns targeting their children. They are often very afraid of publicity.
Furthermore, from my experience, if you around some of these people long enough, you will discover the unpleasant truth about how they made their millions. Many of them I would call “not nice to be with” types.