One plus one is two. But such is not always the case when one is calculating local and national GDP data in China.

After the National Bureau of Statistics on Monday unveiled economic data for 2013, what caught the most attention was not only the 7.7 percent annual growth figure, but also a peculiar math problem.

China's GDP amounted to 56.9 trillion yuan ($9.4 trillion) in 2013. However, the total of the provincial GDP figures, which were independently calculated and released, was about 2 trillion yuan more than the 56.9 trillion yuan that the NBS arrived at, even though three of the 31 localities that had not yet released their totals were not included.