I have not heard this problem before, but this could definitely happen. The OG goes through a wide range of CR questions, all the way from the very easiest to the very hardest. Generally they get tougher the farther you go along. So if you are getting 80% - 90% correct, just remember, a lot of these CR questions might only be seen by the lowest of verbal scorers. When I had my test day, almost all of the CR's were quite challenging b/c I had made my way into the harder bucket of questions.
This may be what has happened with you on the MGMAT CAT tests (which I'm assuming you are referring to). You may only be getting questions from the medium or hard bucket, and therefore you are missing more problems. This is ok though, since getting just 75% of hard quesions right is still doing quite well. So I think you should continue practicing, make sure to go over all your mistakes and understand why you missed them, and you will continue to improve. Don't expect to hit 80-90% on any CAT test, it is just unreasonable because if you are doing that well you will continue to face more challenging problems until your percentage begins to drop.
Best of luck with the rest of your studies
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