看了TOS第三集，企业号和外星人的球形太空船相遇，KIRK舰长自称企业号是“地球联邦”的船，而不是“星际联邦”的船。

看看这段：
Presumably, the United Earth Government at the time of Star Trek: Enterprise (who is responsible for the Earth Starfleet) will remain in place until the founding of the United Federation of Planets. The 23rd century existence of a United Earth Space Probe Agency in TOS Season 1 indicates this name persists referring to the world government even after it joins the UFP.

虽然在23世纪Earth已经加入UFP，但还有人会用United Earth这个词。

估计这也只是对剧集前后不一致的一个勉强解释吧。