There are several possible reasons:

 1. China uses Special Trade System (please refer to Concepts and Definitions of IMTS publication for further information about Trade System, see knowledge base), which means that China has free zones. These free zones are normally not part of customs territories and in special trade system trade between free zones and rest of the world is excluded. In special system, free zones is also considered as partner country.

It is possible that during custom declarations, goods from free zones to free circulation area was recorded as goods from China itself not free zones

2. In outward processing, domestics goods from China were processed abroad and re-imported back to China. Because the goods is originally from China, China was recorded as partner country.

 I think that in case of China, no.1 is more likely explanation.

 Your comments are welcomed.