The author assumes a similar stance at all times while the shots were fired. In theory, the arm wounds could have happened, then Saint Mike the Gentle could have stopped, raised his hands - and been shot twice more. You cannot assume the wounds to the arms occurred and then were followed immediately by the other 2 wounds.
The act of running brings the arms and shoulder up and down. Throw in the narrative that Mr. Brown was rushing the cop and the ability to hit such a moving target in a similar fashion under extreme stress denotes a cop that would make tgo proud.