Dr. Horn testified that was a typo. He was crystal clear in his testimony about the damage caused by a shot to the head in this area of the brain
http://jodi-arias.wikispaces.com/Horn+Final
"Your report stated there was no damage to the dura mater membrane. Can you explain how the brain can be damaged without damaging the membrane?"
Horn: I, ah, let me review my report... it’s the usual protocol with the report so clearly the dura mater had to have been perforated so that would be a typographical error in my report.
Horn: A bullet as it passes through something soft like brain, will create what is called a temporary cavity, and you see that in ballistic gelatin when you test fire a bullet through gelatin. You see it in the ballistic labs. It’s the same concept in the brain. You get a very large open area that squashes the brain around it into the skull around it, and then that will collapse back down ... and you may be able to see a wound track, but in this case, we don’t. But it damages areas of the brain away from the frontal lobe, even only passing through that one part of the brain, other areas of the brain are involved and damaged.
Martinez: So if it was the first shot, what is your opinion as to whether or not Mr. Alexander would have been incapacitated?
Horn: He would have been incapacitated.
Martinez: And would this have been an immediate sort of reaction, seconds?
Horn: Yes.
Martinez: By seconds, how many seconds would you indicate?
Horn: A second or two. He would collapse to the ground and be unresponsive.
Martinez: I don’t have any other questions. Thank you.