I have the same problem as yours
Eventhough,
MGMAT is a good test but score can be inflated
by the fact that it is not adaptive test.
many incorrect answers in easy question can be offset by
just 1-2 correct in 700-800 range questions
I experienced that many times
I can easily answer "defined as hard(700-800)" questions correctly
in less than 2 minutes while struggling for more than 3 mins in
500-600 level questions and ending up answer incorrectly
Another fact is if you make many mistakes in medium-level questions
in official test , you will not have a chance to see hard-level questions.
Your score will be limited to lower range eventhough you have ability
to solve hard questions but your eligibility to see them is not offered
anymore.
I score
MGMAT verbal test#3 39
My hit rate on OG11 SC-70% CR-85% RC-78%
but end up score 16 on official test
one more fact is vocabulary level in real test in higher than GMATPREP
and any other materials . If you are not native speaker or really
strong in vocabulary, you may have a hard time solving
RCs and some CRs. This is enough to lower your score by 10-15 points