The probable reason for that is to make the white homicide and general crime rate higher and less dramatically lower than the black rate. (A black rate close to 12x the white rate is harder to hand wave away).
The one homicide rate that few pay attention to is the "unknown race" demographic.
In Chicago, the majority of homicides are committed by "race unknown" yet they take place in black neighborhoods and leave black victims.
They don't get counted racially because they don't get caught and witnesses, if there are any, often won't talk to the cops.
If you were to add 90% of "race unknown" to the black number (and it's probably closer to 99%), we'd find that young black males are responsible for over 75% of the homicides in the United States.