I think the issue is that the Chinese system does not report numbers beyond 500 and it can easily surpass 500.
Also, I find this amusing:
"The existence of the [U.S] embassy’s machine and the @BeijingAir Twitter feed have been a diplomatic sore point for Chinese officials. In July 2009, a Chinese Foreign Ministry official, Wang Shu’ai, told American diplomats to halt the Twitter feed, saying that the data “is not only confusing but also insulting,” according to a State Department cable obtained by WikiLeaks. Mr. Wang said the embassy’s data could lead to “social consequences.”"
Because, telling people their air is killing them is something to be avoided.