It's really not that big of deal. Likely just a lot of about which words are used. With most all evidence there is not 100% true all of the time. The appropriate language to be used in most cases is what was said in the article. "This bullet is consistent with other bullets fired in the gun in question" or "they are not". If an expert is so bold to say without any doubt this bullet was fired from that gun, the expert is putting themselves out on a limb they may not be able to prove. But, bullets aren't the only evidence. Cases have a lot of value also. With bullets used as evidence it was, (I say was because I don't know about today's tests) The expert would have the bullet from the crime scene. They would ID it as to caliber, weight and brand. Then get some of that ammo, fire the same bullets through the gun in evidence. They would compare about 3 bullets and not find 1 thing in common but many. That's easier to say is that bullet was fired from a certain brand and maybe model of gun.