No P.O. Boxes exist on bases. The format is name, office, unit, base, ZIP. Because everyone on a base is there working in an official capacity.
Guard might have been a little looser...but how about this scenario:
"GWB: I want to send you this (....)
Killian: Send it to my personal address so I am sure to see it ASAP.:
Some enterprising media lurker might ask the Killian family if they remember a "PO Box 34567".
If they do, and it was one personal to the LtCol or family, bang - the forger got the box address off the released letter Bush wrote to Killian.