It is actually much complicated than those maps showing only one governing body. Actually those maps show precisely the periods of unity (Han, Tang, Yuan, Ming, Qing) where only one major power governs, as well as the periods of divisions where foreign ethnicity capture a sizable portion of China.

In latter case, the map fails to capture the lost boundary, like in this case: https://en.wikipedia.org/wiki/Jin_dynasty_(1115%E2%80%931234... where Jin dynasty is in control of majority of the territory and the map only shows the defeated Northern and Southern Sung dynasty. Actually the website that you quoted from also has the information:

> brutal invaders drove the Chinese from their northern territory, forcing them to migrate south and establish a new capital city.

http://archive.artsmia.org/art-of-asia/history/dynasty-sung....