1st: As I stated earlier it is much harder to shoot a moving target than a standing one. If it had happened the way that you are stating here then the officer would have been able to hit the torso quite easily, even at 30ft away. Yet according to the Brown family coroner only one shot hit the torso and that was more closer to the arm than anything else.
2nd: Regarding your grazing shot comment. Experts can tell what direction a grazing shot comes from and according to the Brown family coroner all 6 shots came from the front. So Brown couldn't possibly have been running away when that grazing shot hit him. Here's a slight explanation here.
Here is the Brown families coroner's picture of where the shots were. You will note that not one perforation is on the back side of the arms, torso, neck or head.