A few notes: the maps you've shown dates to 750 not to the 660s when the Tang reached its greatest extent after conquering both parts of the Gokturk empire.
After the Tang occupied the Eastern Gokturk Empire in 630, it divided what is today Inner Mongolia into two prefectures and the rest of the tribes north of the Gobi in what is today Mongolia and Siberia were under the control of the Xueyantuo Khanate. The Tang destroyed this khanate in 647 and divided its territory into six prefectures and seven protectorates, two more prefectures were added in 649 when the Guligan north of lake baikal submitted. This marked the northern most extension of Tang power.
The western most extension of Tang power was achieved after 657 when the Tang army destroyed the Western Turkish Khanate and Tang power reached all the way to Sigistan on the border of Iran. The territory of present day Afghanistan was made into the Tuhuoluo protectorate and it was possible that Tang prefecture generals were established there too. This control was short lived and ended in the 670s when the Tibetans and Arabs, along with Turkish rebellion cut off communications between these regions and China.
The Tang already lost its Mongolian possession in 686 after the Later Gokturks declared their independence in 682 and conquered the tribes north of the Gobi under Tang control in 686.
Even the map you've shown for 750 has a few mistakes; the Ordos region was still part of the Tang empire in 750, just not a prefecture; it still had three fortresses known as Shouxiang city on the northern bank of the Ordos. The Kingdom of Nanzhao and parts of Manchuria were all parts of the Tang prefectural system. Below is a more complete map of the Tang in 741 also made by TanQixiang, the most authoratative map historian in china:
Edited by honeybee - 17-Nov-2010 at 05:20