He is lucky and almost always wins his bets.
FYI - Simon Wheeler does not suggest to the stranger that they bet on his frog. Simon Wheeler is the one telling the story about Jim Smiley and his frog Dan'l Webster. Dan'l Webster is the frog that is a great jumper so it's a good bet. However, the stranger fills Dan'l Webster with shot so he can't jump high and Smiley loses the bet.
Shakespeare influence around the England is True