The institutional priorities for admission shift over time as the class takes shape. Given the high volume of applicants, there is no particular reason for a college to hang on to an ED applicant who didn’t make the first cut .
All through the admissions process the admissions committee will be getting statistical and demographic data back as to the cumulative effect of the decisions they have made so far. And that will shift the focus as the admissions process continues.
I’ve always felt that ED could not possibly be a boost for a weaker, unhooked candidate – why would a school possibly want to take in students from the lower end of their ED pool?
But RD may be advantageous for many students if their attributes happen to coincide with the apparent needs of the college as the class is shaping up. These may be students with more lopsided profiles, or students who bring some sort of diversity factor.