Statistics don't work like you think they do. The number is controlled.

If you come to that conclusion, the thing you should do as a rationalist is "notice confusion". Then you would check the source and would see:

While black residents accounted for 67 percent of Ferguson’s population, black drivers accounted for more than 86 percent of the traffic stops made last year by the Ferguson Police Department, according to a report produced by the office of Missouri Attorney General Chris Koster.

If you want to learn the relevant statistical literacy skills to understand what the sentence "Ferguson police were twice as likely to arrest African Americans during traffic stops as they were whites" usually means, the relevant subject is regressian analysis.