It is not that "I think" it is an outlier, it is that it is a statistically an outlier, an attack on the scale that happened in the US (and even worldwife) once in its existence, that lead to orders of magnitude more casualties and damage to infrastructure, and that provoked unprecedented response: first invasion in Afghanistan, then in Iraq.
It is something that likely won't ever be repeated, so counting it when calculating your chances to die to a terrorist attack in your life doesn't make any sense. It is like, I don't know, looking at all nuclear attacks in human history, noticing that the only two attacks were carried out by the US, and concluding, "If I die in a nuclear fire, then with 100% probability it will be the American nuclear fire".