"Arbitrary" is absolutely correct. The tests in the prep book are much harder than the real test. Actually, on one of the practice tests, I did as bad as I did good on the real test (bottom x% on practice test; top x% real test). I hesitate to say this because perhaps if you're a little scared it will make you study more (like it did for me!) however, don't get down on yourself because of those practice tests; they are quite challenging. Good luck!!