Both the sentences are to be served concurrently which means that they must be served at the same time. As per the question, the ten-year sentence and the three-year sentence would be served simultaneously. Therefore, the three-year sentence would be served with the ten-year sentence making it total of total ten years.
I think it was because we really don't know what exactly happened in the past. Also, I'm pretty sure people did not keep track of colored people back then.