Higher income families received fewer A's (under the new system).
They seem to be suggesting this is because students from "higher income" families were better able to adjust their behavior to work with the system, including lower absenteeism, turning in assignments on-time, and completing extra credit assignments. Oddly I was one of the lowest income students in my high school, and yet I managed to meet these minimal standards!
Oddly enough, these same behaviors (showing up, on-time-delivery, and going above-and-beyond) are also desirable skills for nearly any employer, thus "Grading for Equity" does a disservice to students by explicitly removing them from the grading equation
Looking at Joe Feldman's defense of GfE, the one thing he seems to get right is his suggestion that a student's grade in a given course should be most influenced by whether she mastered the subject matter at the end than how muchs he struggled with it at the beginning, middle of the term