Let's face it. Your board scores will determine in large part whether you are competitive for certain specialties just as your MCAT scores determined if you were competitive for certain medical schools. Students who go into college or who go into medical school with prior performance that was exemplary are, on average, more likely to do well in the next phase of their career than those who had prior performance that was marginal. Schools that take only students who were exemplary in undergrad are, in all likelihood, going to produce MDs who did well on the step exams and distinguished themselves in clinical clerkships. Schools that take highly motivated students who were not stellar test takers in undergrad and on the MCAT are likely to produce graduates who can pass the boards and be licensed but who do not rise to the top 5% of all MD grads.
Now, do you think it is the schools that produce great match lists or the raw material that enters the school is better at some schools than at others?