Why are some of the the bluebook practice tests so vastly different in terms of difficulty

Ok so I took practice test 9 on bluebook yesterday and I got a 1540. It felt pretty light to me. But then I took practice test 7 today and the RW seemed many magnitudes more difficult. It's so frustrating. I ended up getting a 1420, which is not what I am currently aiming for. Which practice tests on bluebook truly represent the difficulty of the SAT and how should I modify my study routine to accomodate a potentially harder SAT exam? I will be taking the SAT this september