This is because of the instant buy sell pressure of that exchange. Sometimes a exchange can have a big buy and sell order with low liquidity, which drives the price ups and downs more than other exchanges. Actually this price difference doesn't last for long, as any arbitrage trader may find it and take the opportunity. They also sometimes exchange themselves to fill the gap to match the price of the global market. But this is normal to have a little spread in price compare to the other exchanges.