The implication being that there could have been any number of shootings in the same calibre that were not effective when only a single hit occurred and multiple follow-up shots were necessary. By only looking at the cases where a single hit was recorded, it seem obvious that the effectiveness of that calibre would be extremely over-rated.
Lets take a very simplified situation as an example.
Lets say that we have 10 torso shootings with the XYZ calibre. In 3 of the cases a single shot was fired and the attacker dropped instantly to the ground. In 6 of the cases multiple shots where fired. In one of the cases a single shot was fired, but the attacker was close enough to knock the gun out of the defender's hand, and do considerable harm to the defender. First, lets look at the total situation.