The tradeoff between economic growth driven by energy use (for production) and pollution.
"The environmental Kuznets curve is a hypothesised relationship between environmental quality and economic development: various indicators of environmental degradation tend to get worse as modern economic growth occurs until average income reaches a certain point over the course of development.[12] Although the subject of continuing debate, some evidence supports the claim that environmental health indicators, such as water and air pollution, show the inverted U-shaped curve.[1]
It has been argued that this trend occurs in the level of many of the environmental pollutants, such as sulfur dioxide, nitrogen oxide, lead, DDT, chlorofluorocarbons, sewage, and other chemicals previously released directly into the air or water."
From what I understand, much of the pollution comes also from other industrial cities and accumulates in this general area of the country (someone that knows more about it please correct me if this is wrong).
The government seems to be taking some actions, but it is hard to know how hard they're tackling the problem.
So sometimes the figures are the same, but that usually only happens when coming down from the peak of a particularly bad day, when the average from the previous day's readings are pulling up the official Chinese numbers while the US figures have already fallen to reflect current conditions.