主要解释在于服务业的多样性，以及相互承认、或者叫做“原产国原则”对服务业的影响，而“原产国原则”在产品市场的联合中至关重要。
Much of the explanation lies in the very heterogeneity of services and the greater difficulties that mutual recognition, or the “country-of-origin principle” – essential in the integration of product markets – implies for services.