It was probably set up that way because the neighborhood or at any rate the area from which the magnet school drew its kids was mostly white in 1974, and the court wanted to make sure there weren’t “too many” whites. That has no doubt changed in the 30+ years since the court decision and it’s probably getting hard to come up with the “quota” of whites.
Every time I read about this racial and ethnic tinkering, it makes me mad. It’s completely wrong and does no good at all for the kids.