What's the difference between Jiangnan in the Southern Dynasties described in Song Shu and Jiangnan in the Han Dynasty described in Historical Records by Sima Qian?

Historical records show that during the Western Han Dynasty, the area south of the Yangtze River was sparsely populated and undeveloped.

During the Southern and Northern Dynasties described by Song Shu, the economy of Jiangnan developed rapidly, Jiangnan was developed, and the people were rich and comfortable.

The reason is:

(1) Jiangnan region has abundant rainfall, hot climate and fertile land, which has favorable conditions for developing agriculture;

(2) Since the end of the Eastern Han Dynasty, many people fled to the south of the Yangtze River to escape the war. The first large-scale population migration in ancient China since the Western Jin Dynasty brought a large number of labor, advanced production technology and different lifestyles to the south of the Yangtze River.

(3) There are relatively few wars in Jiangnan, and the social order is relatively stable.

The hard work of the people in the north and south has made great contributions to the development of Jiangnan.