Well, it is subjective to an extent. Some students who got a C at AS were getting As all year, had a bad day, and are retaking that module, so can realistically get an A. For others, they'll be lucky to stay at that C. It depends on the student.
Our school tends to predict a grade up, since it's the second year of sixth form, you're into the swing of things and will learn from the mistakes you made at AS. Although BBBC predicted AAA does seem a little weird to me. Although if the Bs were really high Bs...