(Based on the diagram from this autopsy report, it would appear the officer started out firing low and worked his way up.)
But if Brown had his hands up, his palms would have been facing out, as shown in the autopsy sketch. Just imagine if the arms are up in the sketch instead of down at his side. Then all the shots would be fired high.
Grazing hit on thumb, bullet track is towards elbow.
Meaning: hand was outward, as if grabbing.
It is, of course, possible.
However, when firing a handgun, the tendency during the firing of repeated shots is for the muzzle to climb upwards.