It does seem very odd that there was no difference between high-quality and low-quality black candidates, but I'll guess at an explanation: employers are suspicious of black achievement because of affirmative action and similar programs. It's harder to know if a black applicant "earned" his accomplishments or not, because they're more likely to be given undeserved grades, accomplishments, opportunities, etc.

And I think all you need for evidence that this is true is ... this study! The failure of an employer to interview enough black candidates is considered by many to be prima facie evidence of racism. (Just look at the comments here. Likewise, giving more failing grades to black students is racist.) So-many people are desperate to avoid looking racist, and black students/employees/etc. get a lot of little advantages as a result. Some of these are harmless (well performing black guy gets first crack at a new position, rather than well performing white guy), but some of them aren't (now-poorly-performing black guy doesn't get fired/demoted from new position).

Or everybody's racist.