Let’s find out.
According to
http://news.harvard.edu/gazette/story/2015/04/harvard-college-admits-1990/
H accepted 1990 students in April. And
http://www.thecrimson.com/article/2015/5/15/class-2019-yield-81-percent/
said that the initial yield was around 81%, so there were Int[0.81*1990]=1611 students decided to attend before the waitlist admission.
From
https://college.harvard.edu/admissions/admissions-statistics
There were1665 matriculates in total. This means that 1665-1611=54 matriculates were from the waitlist. There were 93 admitted from the waitlist, so the yield for admitting from the waitlist was 54/93=58%.
Admitting fewer in April would let the school claim a lower initial admit rate, but it would cost so much trouble to go through waitlist.
Admitting more in EA would achieve higher yield, but it could cost the school total applications.
S has been doing the right thing ~ being consistent each year.