I have E-mailed GL Assessment and their answer is :-
"In constructing the Pack 2 tests, we paid careful attention to balancing levels of difficulty, so that each subject test within the pack is at a comparable level, with a good mix of different levels of difficulty across each test. A similar process will have been carried out for the Pack 1 tests.
How your daughter performs, however, depends to a degree on how easy or difficult she finds certain question types or topics and whether these appear in the relevant practice tests. This is an issue for NVR, in particular."
So the answer is ... Keep practicing!