The 15 percent is likely explained by court considerations like previous record, no job, not at college, alrready on bail, no remorse etc. In fact 15 percent is low considering.Hmm. A little of both. Blacks were 15% more likely than others to get jail for the same offense. Maybe there is a reason why, maybe they were 15% more likely to scowl at the judge and roll their eyes. Maybe it is real bias. Who knows.
But 15% is only a small part of their overrepresentation. It would also appear they are more likely to do the crime.