If I remember correctly the predicted grades on based on such questions as your is your parents job and what was the highest education your parents did - NVQ 2/3/4 GCSE, A level, Degree, Doctorate etc etc. At the time I felt quite cross about this as I was a single parent and a childminder, but my son's best friend had two parents, one a deputy head and the other a physiotherapist. Which presumably predicted his grades higher than my son.
Anyway when my own son went off to Uni it didn't really matter. (It might be the predicted grades for A levels tho - not GCSE - cant remember