@scorpion.ks:U r right.Not all of the questions from GMATPrep were new ones. So, probabaly that does make a difference.
But as far as I know and have observed a couple of people on the forum, the guys are really gud.
If I take my example, I have got even the most difficult CR's on this forum right and on average most SC's right
I have observed Retake_GMAT's answers as well, the CR explanations provided by Retake_GMAT have been awesome!!
So, there does not seem to be an explanation for scores of 540 and 610!!