It seems paradoxical that a student be expected to achieve a higher grade in Further Maths than in the more common Maths exams - and it is. The reason our usual predictions seem to show this is down to the set of students taking each course and included in the calculations used to make the predictions. It is unusual to find a student taking Further Maths who has not already demonstrated their ability in Maths, so their predictions in both will be high. Generally though the predictions in Maths are moderated by the students of less ability in the subject.
Our predictions are normally based only on the GCSE or test baselines of all the students taking a course. Given the extra information that a student is taking Further Maths will allow their Maths predictions to be estimated separately, and as you might expect these will be higher than their predictions in Further Maths.