I suspect the 'affirmative action' trend also reflects the difference in ambition/drive between the low-income high achievers and the >70% high achievers. The low income group is smaller than the >70% group (look at the size of the 'above average odds' regios and the 'below average odds' regions). This is because most of the people in the low income group will never even apply for a private college -- only the strongest applicants apply. But from the >70% group, basically every kid applies to college, and they are much more comfortable applying for 'reach' colleges (even if it costs their parents a few hundred extra dollars). They've also gamed their test scores.