It remains all superstition to me, one of the many superstitions surrounding gambling. If a madman can give correct predictions for a game, why has no shaman or prophet been able to give correct score predictions for several games?
Most times the people who tell this kind of story always claim to have heard it from another person; it is rarely a firsthand experience.
I can't help myself with what I'm thinking right now. Perhaps, the insane man was once a gambler who runs mad due to his excess gambling. As you said, story of this nature is rarely a firsthand experience because I've also heard several things like this in the past.
As for prophet, I have witnessed some few scenarios where they predict a game and lost. I realise there's no special communicator with prophets on what is going to happen in the future. They relied on assumptions just the way we predict and fail at times. Once trial accurately doesn't mean one is a seer or perfect.