Maybe you are talking about the difference in the h-indices of China and the US? Because that's a more reliable indicator for quality over long period of time.
As a matter of fact, there's not much of a difference between the citations per document of the two countries. It's 0.80 for China and 0.86 for the US. Pretty much the same. And citations do not necessarily translate into higher quality articles, although there's no doubt that the quality of research in the US is higher than China.
In fact citations per document is one of the worst indicators for measuring quality. It's not robust at all, and it's completely unreliable. Vietnam has a higher citations per document than the US. That doesn't mean the quality of science produced in Vietnam is better than the US.
That aside, it has been independently verified times and times again that scientometrics cannot be based on raw data like the number of citations only. And there might be no good indicator to instantly measure the scientific impact of a country. There are scientometric measures that are work well over a long period of time, but in short intervals of time like a year or two, no scientometric indicator alone means anything.