“per capita”
This is a statistical term meant to signify that this is an average based on the number of residents in the area. DC is number #1 because of it’s large amount of murders compared to the small number of residents.
If you’re going to compare crime statistics, most statisticians divide the number of a crime (homicide) by the city population then multiply it by the largest unit of measurement they each have in common (usually 100,000).
I would guess it’s because there are a lot of people who don’t live in there city but commute there for work