there is a upfront exchange rate rebate or commission earned by the acquiring bank of APPLE and HSBC, usually the commission rate may go up to 2%..so it is natural that the exchange rate at internet buying using foreign currency is higher than usual exchange at the money exchange.
there is a HXBC USD credit card, such that you can settle your apps bill at USD, so there is no exchange rate loss..besides, you can use a reasonable exchange rate to settle the bill