I think PR doesn't have a great scoring algorithm. i have one quant test where I missed three total questions and got a 42 and another where I missed 8 and got a 48. I know that it is based on how many higher level questions you get right, but their scoring is all over the place. On GMATPREP, I missed 12 and got a 48.
I also feel that a lot of the CR and RC questions on Princeton are too vague to be on the real GMAT, and this may skew your score. Obviously, GMAT PREP is the best score source, but I have read that Manhattan is the second most accurate on some other sites.