A lot of people forget that the population of China is what, 1/5th the world's population?
As such it would make statistical sense that around 1/5th of attacks they see are from China.
This is a figure that tallies roughly pretty well with attacks I've seen on every net facing system I've bothered to monitor. I wouldn't say there are proportionally more attacks from China relative to their share of the world's population than anywhere else. Given the US' population, Russia's population, or a number of South American and Eastern European states whose names I've seen popup a fair bit it's actually the case that I see disproportionally more attacks from these states relative to their population.
That correlation doesn't hold, I think. A more appropriate one would be to compare learned users of each country's population that can access the Internet. My understanding is that the majority of China is poverty-stricken and not using the Internet. And by this same position, I would expect the cracking attempts from US-based locations to vastly outnumber all other states in sheer number, but I don't believe that's the case either.
Another poster had the right angle, I think. The number is greatly influenced by state-sponsorship or lack of law enforcement.