Discrepencies in Political Futures Markets

On The Iowa Electronic Market, a predictive futures market ran by the University of Iowa, the contracts for Obama winning the Democratic Presidential Nomination is trading at .928 on the dollar. On the Intrade Prediction Market, the very same contract is trading at .948 on the dollar, or a 2.2% premium to the IEM contracts. Why the discrepancy in prices? Both contracts pay out at the same time, August 26, and the same risks? Arbitrage Opportunity.