It's all about the yield.
I'm surprised nobody has mentioned the practice of city kids testing at Catholic schools as a Plan B if they aren't accepted into CPS selective enrollment schools like Young, Jones, Payton, Lane, Northside Prep, etc. Roughly 1 in every 6 applicants to selective enrollment high schools are admitted to one.
From the Lane Tech website: "Each year approximately 1,200 freshmen are selected from more than 13,000 applicants." I wonder how many of the 800+ kids who tested at DePaul Prep have also tested for admission to Lane (both of which are separated by all of 1,106 feet) or Payton or Northside Prep, etc...