I have heard that too, but have not gotten to 6 yet. It might be all the high yield topics are being exhausted and they are getting to the lesser known MCAT topics. I hear Kaplan and TPR FLs are structured like that since they have so exams many compared to EK.
If you keep testing the same material over and over, you won't learn as much and the scores will be less predictive. Everything on the AAMC outline is fair. That's why the EK exams, while good, are not nearly enough (and not for $50 apiece).