Well, that problem is two fold.
Affirmative Action admits are based on the potential of the steudnet to do well in college. However, colleges often receive funding based on admission rates and not graduation rates.
So there is less emphasis on help those students with that potential staying/keeping them in school.
Now, once you change that focus, and attempt to put forth an effort to nurture those students, you see higher grade and a higher graduation rate amongst under-represented students.