Hi Danylopogrebnyak,
Every practice CAT that you could possibly use has some "biases" built into it (even the 4 Official CATs from
www.mba.com come with limited pools of questions). As such, none of them is a perfect match for the Official GMAT, although many tend to be 'close enough' to give you a reasonable assessment of what you could score on the Official Exam. A Q51 with 11 mistakes does strike me as 'suspicious', so it's probably a good idea to take another CAT from a different source. In that way, we'll have a basis for comparison.
Have you tried any of the CATs from
www.mba.com? If so, then how did you score? If not, then you can download 2 for free.
GMAT assassins aren't born, they're made,
Rich