Perhaps not a surprise about that regarding some elite private colleges, since students from the lower half SES (approximated by Pell grant) may make up only 15% of the students (and not all of these students would consider their families “poor”), while students not getting financial aid (probably means family incomes over $200,000 per year) make up about half of the students. So those colleges are places where high SES predominates, with student services and such aligned toward high SES.