I once asked a person on an admissions committee why minority applicants are screened/reviewed separately from the rest of the applicant pool, often by the same person and in this conversation, this person mentioned that studies have shown (I don't know exactly what studies) that even though some minorities may be admitted with lower scores that there is no coorelation with how they perform in medical school. Basically, he was saying that they are just as competitive as their other classmates.
I honestly don't know if they look at non-minority applicants with lower test scores as a risk, but seeing other applicants with higher scores overshadows someone with a lower test score. Basically, someone with a higher score has more to offer as an applicant. You occassionally see or hear about people coming in with low test scores if the rest of the application is super tight (ie. they have something to offer), which makes me think that a lower score doesn't necessarily mean risk in their eyes. A lower score means this student might have less to offer.
A minority with a lower score has
1) added diversity to the class (you may argue the necessity of this, but studies have shown that students as a whole learn better and are happier in more diverse environments)
2) minorities are more likely to practice in underserved areas, something else they are able to offer.