The theory being that Impa is the name of every (female?) leader of the Shiekah. Not so much that the name itself is passed down, which it obviously is-- just like Zelda. It's been the name of each of Zelda's caretakers but I'm pretty sure Impa isn't the leader of the Shiekah in every one. After she became a sage in OOT, she had other responsibilities anyway so it's actually quite likely they would have elected a new leader. They probably did it after she was elected to be Zelda's nursemaid actually. It's unlikely she was both at the same time.