Actualy, i don't think it's proportional, because people are individuals, and they don't act like a collective mind. If you follow a strict mathematic logic, there should be 12.6% black people everywhere (police, governement, army, bus driver, janitor, CEO, etc).
But it doesn't change the fact if you are a minority, you will have harder time to get to what you want, especialy if it is a heavy competitive field: you'll have plenty of opposition, and having 12.6% percent black people in a heavy competitive field doesn't mean you'll have 12.6% black people at the end, because that's not how reality works, it's like flipping a coin: you statisticaly have 50% chance to go on either face, but you can still go on one face 100 times.
Or to give another example, let's say you have an annual examination, one year, you'll have 12.6% black people, ok? One year later, you have 30%. At the end, that doesn't mean you will have 12.6% or 30% winners.
It's like asking why the field of rap and r&b is almost exclusively taken by black people: in statistical terms, it's an aberration, in terms of reality, it's fairly logical: does it prove that black people are actively trying to prevent white people from enter the field? Or does it mean the white people are simply not interested by the subject?