Hi,
In my opinion, our score in GMAT Prep exams is usually exaggerated, since we have already done a few questions earlier. This not only helps in getting these already done questions correct, but also saves times for other questions, which we get correct since we have enough time. This boosts our score to a great extent.
I scored 650 and 680 in my GMAT Prep 1 and GMAT Prep 2 respectively. So, I went to give the official GMAT and score a meagre 540. I was highly disappointed, but then I realized that in the actual exam all the questions are different and a bit more difficult than the GMAT Prep. Hence, a drop in score is expected. This drop varies on a number of factors. My belief is that the score in the actual exam is the indicator of our true score and performance.