Duke defers about 18% of the ED class (903 / 4,852) which is almost the same number of ED acceptances 882. Of the 903 that are deferred to RD, about 10% actually get accepted versus 5.7% of the general RD population.
Just a general comment and speculation but I’m pretty sure that accepting a larger % of deferred applicants is important as Duke knows this was these students’ top college choice (from ED) and their yield is going to be much higher if accepted than the general RD group.