Coach wrote:
I believe the reason for the APU deflector was to lower the possibility of post shutdown fire from residual fuel in the left engine when restarting the APU. Could be wrong...it also may have been purely cosmetic when they switched to the gray paint.
In other words...I don't know.
Coach
Not too sure how a small lip deflector would minimize post shut down fires. Not even sure the small deflector would shoot the APU guts to the ground if it grenaded. I'm sticking with the other explanations...to keep soot off the cowl/cosmetics.

At Hill AFB when I launched one out Thursday in 0F weather, it didn't seem to deflect much of anything. I stood about 15 feet from it to keep warm and it seemed the exhaust hit me just fine to keep me warm as if it didn't even have a deflector.

The deflector deflects the hot exhaust away from the fuel drain at the bottom of the nacelle.