@Thiron The outer court is official business area so like Maomao said they are secretaries, not handmaids, the girls in the inner court are, for lack of a better word ( more elegant at least ), breeders. Their principal "utility" is to procreate, so even if there is the upper echelon between them that are "high" born some of the intermediary levels of them can have girls of different walks of life, hence different levels of education and "qualifications".
It is a messed up system? Undoubtedly. But in that age, it was the norm so to speak.