The NRDC said their second report was based on "newer, more reliable figures". [1]
The major difficulty in calculating Chinese CO2 emissions was that China was massively restructuring its coal and petroleum industries in the late 1990's making year to year comparisons very difficult. In addition, China was also imposing new coal mining safety standards and cracking down on illegal oil importation. Both of these meant that there could have been a large amount of fossil fuel burning which was not included in the official statistics.
See: global warming, Kyoto Protocol