According to Yukirin the term was actually born because she was not included in it.
At that time Yukirin ranked 9th in the first senbatsu sousenkyo and ranked 8th in the next sousenkyo.
Simply put, Kami-7 to refer to the most popular members and differentiate from other members whose popularity considered average (including Yukirin who is not considered Kami-7).
The original AKB48 Kami-7 were: Atsuko Maeda, Oshima Yuko, Shinoda Mariko, Watanabe Mayu, Takahashi Minami, Kojima Haruna, Itano Tomomi.