My sons independent has a similar system, they divide into quintiles, so a child in the 5th quintile will be in the bottom 20% in the class.
At secondary level I honestly dont worry too much if he is in the 5th quintile in a subject or two. He is in a selective independent where they all had to pass 11+ exams to get in, and interview well. He came from the state sector where he was Level 6 for maths, where national average for leaving primary is Level 4, most kids in his class achieved a Level 5. He is in the second quintile for maths, achieving around 80-85% in tests, so not in the top but doing well.
The point is, when they all have to pass exams to get in, and there are 600-700 applicants for 100 places, and only the top 100 applicants get in, it goes without saying that the level is high, and to be in the bottom quintile is still not bad actually, he has years to catch up.
If your daughter is in the bottom quartile, she is in the bottom 25% out of a class of children selected for their abilities, and after tutoring, it means she still has a massive advantage educationally even if she remains in the bottom quartile. As somebody else has said, somebody need to be the bottom 25%, and if the level is high it wont matter.
Many of the children from my sons secondary prep school did not get in to the schools secondary. Many of the parents were irate that their children had not been taught to passing level; it did not reflect good on the school.
If your dd is really struggling, and you feel she is not learning, she possibly is not in the right school for her. If she is still progressing, but at the bottom, I would not worry. She is still young. At her age, she needs to have other stimulation than just formal learning to fire her imagination and allow her to develop and mature, like free play. I recommend she collects beetles, jump in puddles, read fascinating stories, go to museums, sing, dance, listen to good music, and climb trees more!