The thing is, I honestly think an intelligent doctor could fail the exam 6 times if English were not their primary language, and that their test score might improve as their English skill improved, as a result of them actually being able to apply their knowledge due to actually understanding what the questions are asking. I went to school with a girl like that in college- she was fresh off the boat from Africa. Year one, we didn't understand two out of three words she was saying. Within two years, she was damn near proficient, and nowadays she doesn't even have an accent.
Maybe the large number of failures is not due to them overestimating their medical knowledge, but rather overestimating their understanding of medical English repeatedly due to having no real opportunity to practice it? I mean, you can practice regular English just by living in an English speaking country for a bit, but you'd have to spend a hell of a lot of time in a hospital to master medical English if you earned your medical degree from a school that uses a different language entirely.