Because banks need to consider many factors such as currency exchange risk and inventory cost when conducting foreign currency transactions.

For the purchase price, there is also currency exchange risk, because holding a large amount of foreign currency inventory will generate capital costs (such as interest). Therefore, when purchasing foreign exchange, banks will give a certain degree of discount or preferential treatment to balance these costs and obtain higher returns. On the selling price, it includes the cost of the service provided by the bank itself as an intermediary. These expenses, including handling fees, storage fees and other miscellaneous fees, are included in the selling price.