
I think its an attempt to show lives_saved v lives_lost; all other
 gun related crimes don't result in lives_lost.  On the other hand,
 its impossible to know how many of the successful self defenses 
 prevented lives from being lost.  In other words, the lives_lost
 is pretty clear [its the homicide and non negligent manslaughter
 number], while the lives saved is some percentage of the successful 
 self defenses.  Clearly that percentage doesn't have to be real 
 high to show that lives_saved > lives_lost.

As a semi-related point, check out Kleck's "Point Blank".  I believe
 it goes into some related areas; it also is well written and informative. 

