I am an experienced secondary teacher and Head of Department.
Your DD needs to have a conversation with her teacher about how the grade was calculated. Predicted grades are supposed to be discussed with the pupil and it is not good practice for them to be handed out without a dialogue. I am surprised that your DD's school has just given them out without a contextual conversation.
Predicted grades aren't just calculated on what your child achieves in a mock exam. The teacher will also consider the quality of class and home work completed so far and the level of knowledge and understanding they show during lessons. A brilliant mock exam as a result of cramming is not necessarily reflective of their actual ability and a one off good performance cannot be relied upon to be repeated. So we have to look at the big picture.
I also consider how pupils cope with exams when I predict. If I know a pupil is likely to become very nervous in the exam room, go blank, write a load of nonsense, etc, then I will be more cautious with their predicted grade as I can't guarantee they will perform to their true ability on the day.
Also, if this is about being predicted an A star rather than an A, then you need to understand how difficult it is for teachers to predict A stars. The grade boundaries shift every year as the % required for one is based on the whole cohort's achievement in that exam session. Unless a pupil is consistently brilliant in EVERY element of the syllabus and I am confident their coursework is of A star standard, I would be very reluctant to give an A star as a prediction. Some Headteachers even actively discourage teachers predicting them unless a pupil really does need them for a Oxbridge application, as it sets up very high expectations that could be problematic later on.
In short - your DD needs to speak to her teacher. She's in sixth form. She needs to be taking charge of this herself, not having her parents email in before she's even attempted to find out why the teacher has made the decision they have.