The reason was simple: businesses benefit from tax exemptions or reductions for products that are exported. So, when companies from the mainland dealt with each other, they would export to Hong Kong first and then import back to the mainland, resulting in the transaction being treated as an export.
原因很简单：企业从产品出口的减免税中获益。因此，当大陆公司发生贸易往来时，它们会先出口到香港，然后进口到大陆，而这笔交易会记为出口。