First of all, the center line that shows the combat results (e. g. "Shrapmetal hits Longfang Witherhide for 5 points") shows only the last result. That means that when you hit him with 7 pieces of Shrapmetal, the 7 messages come one after another and you see only the last one.
Another thing you should know is how the resistances work in this game. I'm not sure about Longfang's stats, but let's assume he's an ordinary Red Dragon, which has a physical resistance of 50. The resistances work like this: first, if your resistance is R, calculate P = R/(30 + R). In the case of the dragon, P = 50/80 = 62.5%.
This P is the probability to cut the incoming damage in half. So if you use an Implosion which rolls 80 damage, there is 62.5% chance to reduce it to 40. But that's not all: if the damage is reduced, there is ANOTHER roll to reduce the damage, and you get to roll either until you fail, or until you reduce the damage to 1/16 of the original amount. So in the dragon's case, there is 37.5% that it takes the full 80 damage, 23.4% that the damage is reduced once to 40, 14.6% that it is reduced twice to 20, 9.1% that it is reduced thrice to 10 and 15.3% that it is reduced four times to 5.
This weird mechanism has interesting effects. It, for example, explains the following scene that seems to occur quite often: you blast a monster with Incinerate for 150 damage (you had a good roll and the opponent failed to resist). Then you do it immediately again and it's only for 5 damage (your roll sucked and the opponent rolled 4 successes on resistance). That's just how the life goes in this game.
By the way, I don't like Dark Magic too much either. I tried Shrapmetal against some dragons, and one cast wasn't even enough to bring one down. (At dark magic level of ~30.) The math does work out, but that doesn't make the disappointment any smaller.
And ad P. S.: I'm sorry, but there have been lots of such claims in the past and all have been refuted by more diligent experimentation. It's a bit like all the claims that in order to have the best enchant results, you need to do it in the evening when it is full moon or whatever.