I don't agree with your logic of incorrect questions. I have got a 39 on verbal on GMATPREP with 12 incorrect questions.
I have got a 40 on verbal with 8 incorrect and a 38 on verbal with 13 incorrect,when I took it 1 month back.
I must add that on GMATPREP I got only 2 SC wrong and 2 CR wrong.Also in Rc I got all main point and purpose questions right.I got most of the infer questions wrong on GMATPrep.
So you could probably infer that I literally killed two sections in the verbal so I wasn't penalized that much.You on ther other hand might have gotten an even split -4 SC 4 CR and 5 RC .This split is worse than mine because my split shows that I have far superior SC and CR skills than you have.This,alternatively,means that the algorithm will throw at me SC questions,which,many a time, are back to back, that are much tougher than yours'.If I keep on getting those questions right,the algorithm will award me for that.
I agree that on some questions in MGMAT CR the correct answer seems a bit far fetched as compared to that in GmatPrep.Same can be said about question stems of the CR questions,which are sometimes ambiguous.I also agree that the correct answer on RC seems a bit far fetched.Gmat requires you to infer very little (in RC questions).Literally,the correct answer is stated in the passage, but MGMAT RC infer questions sometimes require you to make,according to me, a far fetched inference.
These are the only thing that I can think of are affecting you score on GMATPrep.You just aren't used to the real test's format in the sense that you are solving for X when the question might be asking for Y.
Many people make a big mistake in CR that they straight away jump into the premise without thinking about what the question really is asking,but since you have gotten used to those stems you get those questions right.Many times MGMAT uses the term support for meaning strengthen,depending on the direction of the argument obviously.But the GMATPrep explicitly,most of the times,states strengthens.The GMATPrep states explicitly inferred/must be true for inference questions,but MGMAT still uses the term support.
These things might be causing you to get questions wrong,which you should have gotten right.
Therefore,the GMATPrep awards you a much lower score.
Honestly speaking,I would say GMATPrep verbal is of the same level as MGMAT if not easier.The only difference is that GMATPrep uses a more crisp and clear language than MGMAT.
1 more point that I must add is that in MGMAT the questions in verbal are significantly tougher (700-800) when you get them right,that is,if you get a 80% question right,it will straight away throw at you a 700-800 question and if you this too right,your percentile eill shoot to 90%+.
Whereas in GMATPrep,the questions virtually are of almost same difficulty.The GMATprep inference questions in CR,especially,are way easier than MGMAT CR questions,which are next only to LSAT CR questions.
As a result a person,who gets say 10-13 questions wrong on MGMAT,will get much fewer questions wrong on GMATPRep.
I don't know whether you see assessment reports,but I definitely do.You will notice in that the number of questions,especially CR,are almost the same as the number of SC questions and RC questions.
So if you are really good at CR and RC in MGMAT CAT you could,possibly,still achieve a good score.But in GMATprep almsot 17-18 questions are SC,13-15 RC and barely 11-13 CR.
If you are really weak in SC,this weakness will be very apparent in GMATPrep because at same % you will get much more questions wrong.Since SC ,as commonly believed,is the easiest to improve on,getting a SC question wrong,which moost of people would have gotten right,would severely hurt your score.
Also,it is commonly known that the toughest section in verbal to improve on is RC,getting a RC question would shave off much smaller points from your score.
