Abuse of percentages

From an article at the New York Times, about the persistent racial gap in high school test scores:

Despite gains that both whites and minorities did make, the overall scores of America’s 17-year-old students, averaged across all groups, were the same as those of teenagers who took the test in the early 1970’s. This was due largely to a shift in demographics; there are now far more lower scoring minorities in relation to whites. In 1971, the proportion of white 17-year-olds who took the reading test was 87 percent, while minorities were 12 percent. Last year, whites had declined to 59 percent while minorities had increased to 40 percent.

I do not think that first percentage means what the journalist thinks it does...