>Given the big gap between the east and the west

I thought the reason that China and Singapore were so far ahead of everyone else was that they were urban populations vs everyone else include rural areas.

I assumed that Beijing, Shanghai, Jiangsu, and Zhejiang were purely urban, but that doesn't seem to be the case.

I'm very curious about this as well, because BSJZ is almost an entire proficiency level above (almost) everyone else. That's an insane gap.