I'm sure this is due to the HUGE economic gap that exists in DC. You have the poor blacks and the rich whites and almost no middle class of either. Most other cities do-- (Baltimore has a middle class, Philadelphia does, etc.)
Well, that's why we win-lose with the #1 slot. We lack a large middle class in the District.
However in the cities you cite and other cities with a middle class (Chicago has an enormous black middle class, for example), the problem is the same. From the study, verbatim:
"All 21 cities in the study displayed a difference in performance between whites and blacks and between whites and Hispanics."
So while I agree with the point you made about HHI, there's most definitely a CULTURAL component here that is persistent and pervasive.