Um, both? Success rates account for not so much deadliness but more so preparation effectiveness and the will of those taking it. Basically, it's a pretty biased way of assessing whether or not something will kill you. Even if there are outliers like failed SN attempts that did everything right, those are less common than other reasons and make up a tiny pattern of the percentage.
N is more deadly according to statistics because those willing to go out of their way to get N know what they are doing/have a strong enough willpower to go through the effort to get it.