There are a few reasons for that that I can imagine:
- China is one of very few autocracies that has managed to significantly improve the standard of living of most of its population.
- The public trials and (sometimes) executions of allegedly corrupt individuals might help improve the perception of corruption.
- The same harsh penalties mentioned above might influence people to declare a higher level of social trust than they actually have, even if the poll is supposedly "confidential" and "only for scientific purposes".