They aren't all independent 50/50 bets. There are a few clear trends in the wagers (bullish on the US economy, Eurozone/EU will not completely go to shit), so predicting just a few trends correctly wins most of the bets (though being wrong on those same things would lose most bets) and there are some bets at very long odds (e.g. 200:1 odds on Ron Paul becoming president) so even the person on the other side of the bet doesn't think it's a likely outcome.