Hi Kack,
As most of us have found out -- GMAT Prep is bit quirky. I had a definite wrong question in one of the GP I gave and off course a lot of "I dont agree with the ans".
So dont make too much out of it- Just take the score (that is pretty authentic) with a pinch of salt. Anyway the GMATPreps are not good for analysis as there is not answer explanations.
You may find this thread helpful (most probably you have already seen it.)
http://www.pagalguy.com/forum/gmat-r...tml#post289318
Bye
Parag