Well, there's a lack of brains here, but I'm not certain its in the NRA anywhere. The study has to ignore the fact that effectively every state has some for of concealed carry available today, that this trend had begun BEFORE 1999, and that since it has begun (and in fact going back 30-plus year) crime, and violent crime, have been a falling indicator unaffected by the presence of shall-issue concealed carry laws.
The reason for this is obvious to anyone without an agenda. I don't claim to be without an agenda but the reason violent crime was not increased by individual concealed carry was simple: The people who wanted to carry before the laws allowed it simply broke the law. Criminals already did this as a matter of choice, and the people who wanted to carry concealed, who weren't otherwise criminal, made the decision that they'd rather be "tried by 12 than carried by six." So all the people who carried, after the legitimization of concealed carry, were probably carrying before any such law was passed.
In the mean time, violent crime has steadily fallen, and that hasn't changed one way or another as a result of the law.