only explanation for a decline or incline, suddenly, is the amount you are guessing or "leaning" correctly or incorrectly...
if you take a a handful of preptests with 5 sections and use your lowest scoring section and preclude your highest scoring section (worst case scenerio) when scoring yourself then you shouldnt be more than a 4-5 point range.
I can't tell you how many times people tell me they scored a 170 on a preptest then go take the actual test and get 155. Sometimes luck is on your side and sometimes its not by why ever leave it up to fate?