I think I have it.
Consider: It's a given that there are between one million and 2.5 million uses of guns to prevent a crime, and between 50,000 and 125,000 crimes committed with them.
What this means is - the defensive gun uses could all have been defenses against a gun, and those would still count as crimes prevented, not crimes committed. The proportion of defensive gun uses against a criminal with a gun could be 100% or 5%, and it would still count as a defensive gun use, not a crime committed.