I think that's because of the test structure. It is a intelligence test, not a language test in essence. As long as they memorize the vocabs, it wouldn't be too hard getting a 600+. That's why ETS is restructuring the test, which would be much more challenging for the non-English speaking people.
I think most of the Chinese are not comfortable speaking English, but they can read and write. Some of them are actually working way too hard because of the language barrier so they don't have time to socialize with others. But it still seems wired how almost all the Asian students (not including Asian-Americans) are not as social. It just appears to me that they do not belong to the group.