I imagine, if they excluded OTC drug overdoses, the amount of successful attempts would be higher. I imagine a lot of OTC drug overdoses fail. I imagine that may well be one of the most commonly tried methods too, seeing as people have them to hand. They've also likely been mislead by TV programmes and films that make it appear so easy.
It makes sense to me that an impulsive attempt would be more likely to fail. Maybe the person chose a time where it would be much more likely they would be interrupted. Certain methods like SN also seem to work better when protocols are followed. Some methods aren't very forgiving towards impulsive attempts.
That said, maybe some methods are so frightening, that thet require impulsivity to get over SI.
I find the figures weird though. Isn't it something like: 24-5 failed attempts for every successful one? Plus, the even weirder statistic that only 10-15% of people who previously failed an attempt will actually die by suicide. I really don't believe that one. I imagine someone with problems so severe that they attempt to take their life would likely still struggle in life and still contemplate if not try again.