Predicted grades will always be a higher than actual grades for a school, because of the vagaries of exams themselves. If Lucy, say, is genuinely capable, in terms of intelligence, work ethic, and performing at her normal level on the day with a reasonable bitof luck with the questions, of gettinga certain grade, then obviously that should be her predicted grade.
However, she panics in exams (might not do so much in mocks), her dog dies the day before, her mind goes blank, she breaks up with her boyfriend the week before, her period is two weeks late and she is terrified but doesn't tell anybody, she feels unwell (yes I know that's a mitigating circumstance but it has to be proper medical doctor's certificate unwell, not rotten cold unwell), she reads the instructions wrongly, doesn't finish the last question, which can be a big chunk of marks in essay type subjects, then doesn't get the grade.
On the other hand, apart from being lucky with the questions, there aren't many variables that would lead to somebody doing much better than expected.
Maybe that's why there is always a discrepancy?