Hi ronit1894,
The scoring algorithm on the Official GMAT is far more complicated than most people realize. Since that algorithm is proprietary, no GMAT company has an exact match for it, thus CAT scores can vary a bit based on the 'biases' involved in their respective designs. The most realistic CATs available are the 6 from GMAC (the 2 free CATs, and each of the 2 that come with Exam Pack 1 and Exam Pack 2), but the CATs offered by Kaplan, MGMAT and Veritas are 'close enough' to the real thing that those CATs will offer you a reasonably accurate score result (IF you use them correctly).
GMAT assassins aren't born, they're made,
Rich