Its probably a couple of different factors, though I'm by no means an expert. 1. Exactly when do you start WW2? 1939 and the invasion of Poland? 1931 and the invasion of Manchuria? Somewhere in the middle? Adding 8 years and a hot war will change the numbers substantially. 2. China was also in the middle of a civil war; how many of those count? 3. The numbers are always squirrelly in wartime, especially civilian deaths in areas where the records were also destroyed. And there have been strong incentives to "adjust" casualty figures for political ends. Stalin and his successors would inflate casualty counts to show that the West were freeloading off of Soviet casualties or blame deaths they caused on the Nazis, China and Japan try to "out-victim" each other wrt to deaths in Nanjing and Hiroshima+Nagasaki, etc.