My Chinese friends tell me that there is a government requirement that maps databases have an offset. I'm guessing the Google car isn't permitted to drive around in China, because they lease maps from Mapabc [1], which presumably has the offset. If you go to ditu.google.cn you get accurate maps without the offset (but it's only in Chinese).
[1] http://en.wikipedia.org/wiki/Google_Maps (see the "Copyright" section)