Every year I’ve taught there have been 3 or 4 students who smashed it, somewhat unexpectedly.
That isn't what I'm talking about.
I agree that some pupils will always do better, and some pupils worse than expected.
However, there will also always be external non- educational factors that will impact on performance - illness, bereavement, being made homeless, drugs etc.
I can think of external, non educational, factor that would reduce an expected B to an E or a U, but not any that would increase a U to a B. I would therefore expect predicted grades to be higher than actual grades, because no teacher will have randomly adjusted for family tragedy in their predicted grades.
However, if the awarding boards use statistics from previous years to create an algorithm, those factors will necessarily be included - but is that fair?