The NRA summarizes their report in this paragraph:
Coinciding with a surge in gun purchases that began shortly before the 2008 elections, violent crime decreased six percent between 2008 and 2009, including an eight percent decrease in murder and a nine percent decrease in robbery. Since 1991, when violent crime peaked, it has decreased 43 percent to a 35-year low. Murder has fallen 49 percent to a 45-year low. At the same time, the number of guns that Americans own has risen by about 90 million.
Predictions by gun control supporters, that increasing the number of guns, particularly handguns and so-called “assault weapons,” would cause crime to increase, have been proven profoundly lacking in clairvoyance.Documentation is provided at the link. Perhaps criminals perform a subconscious calculation, pondering the chances that an intended victim may be armed and proceeding accordingly. As gun ownership has increased, the likelihood of encountering an armed victim increases apace, and the thug's crude cost-benefit analysis weighs heavily against an assault.
This stands to reason, especially in crimes like rape or robbery. If most potential robbers thought their intended victim was likely to produce a weapon instead of a wallet it could make the crime decidedly unprofitable and just not worth the risk.