oh great one of those "you totally deserve the highest rank but because it would be unfair to have you just so many ranks i'm going to give you a lower rank" situations, god i hate them so much...
there is no reason at all why that's a thing, rank is not decided on seniority it's based on what score you get so why would her score not merit her the highest rank, especially when she's been an adventurer for years and the healer for the "hero" group during all those years.