Remember both men were the same height, if Brown's head were that low while still on his feet Wilson would have been firing from the hip.And the head wound could have come when he "charged the officer with his head down like a football player", as the witness statements read.
The bullet had plenty of remaining energy after pentrating the skull and drilled a pretty straight hole through facial bone out the jaw and into the upper chest, a .38 lead round nose might deflect on the rather thin bone of the skull but I doubt thats what happened here.