Although I dont KNOW, and it is something we as anglers should know, I will offer an idea. The idea is you have a 12lb line with a 15lb fish on it. If you were to drop the fish from a height of 3 feet, it would exert more than the breaking strength on that line. Thus, snap. If you hook a fish (IN WATER MIND YOU) then fight it, you are not necessarily exerting 15lbs of force on that fish. Since it is in water, and you have a drag system working for you, and your rod is bending, all of this taking some pressure off. If he runs and you try to stop him dead in his tracks, then a snap is possible, as all the other factors are gonna add up to breaking point. Maybe even taking 15 pounds and hanging it off 12lb line may not snap it depending on the weight distribution between line to rod, guides, and all that. So even though we are bringing in larger fish than our line can handle, we are not just putting the entire weight of the fish on the line. As I said, I dont KNOW exactly how this whole line test rating vs fishing work, but just a thought. Good question though.