<p>The OP is probably confusing Early Action at Chicago (and elsewhere) with Early Decision. With Early Decision, applicants are committed to enroll if admitted, except for circumstances that rarely get invoked. So 95-100% of students admitted ED wind up attending that school. Most colleges with ED admit 30-50% of their entering classes ED. It would be absurd to admit 95% of a class that way.</p>
<p>At Early Action colleges, including Chicago, MIT, Caltech, Georgetown, and of course Harvard, Yale, Princeton, and Stanford, students admitted EA are not required to enroll. So significantly fewer than 100% of EA acceptees wind up as part of the entering class. At Chicago, it’s probably in the range of 60% who enroll. The target for EA is probably to fill half the entering class (just like ED schools), although in the past few years they may have gotten more than that. So there’s roughly the same room to admit additional applicants in the Regular Decision round that you would find at competitors who use ED rather than EA.</p>