Straight from a director of admissions at a t10 college. You should look into the concept of yield as it pertains to ranking. ED allows schools to secure a certain yield % and have it guaranteed mathematically. Sorry but you're being a bit naive about believing these schools at face value when they say the ED applicant pool is much stronger. Obviously they are not going to tell people "if you're a weaker applicant, apply ED to have a better chance of getting in". Also from my experience in undergrad engineering at a t10 (where I also worked in admissions a bit), where ED applicants were generally much less academically qualified than RD counterparts. It's pretty absurd to believe any statistical normalization can occur across a 20% vs 4% acceptance rate.