Sure. Once the winning number for the lottery is announced, you know with 100% certainty whether you have the winning ticket or not. But before it is announced, we can only talk about probabilities, and I can bet that your chance of winning the grand prize is very close to 0.
One gets actual results from past experience, such as what your mother lived through. But we do not know about actual results for ourselves 40 years down the road, hence we talk probabilities. And no matter what we do or how we plan, there's always uncertainty. The less we spend out of our stash, the more likely we will die rich, but it is not a guarantee.
But I need to point out again that the success rate as shown by FIRECalc does not really apply to past retirees such as your mother, even though FIRECalc is based on past market performance. It is simply because most people do not blindly spend a fix preplanned amount in their retirement while turning a blind eye to their portfolio performance. When their stash shrinks, they cut back on spending, drive less, stop shopping. The result is a reduction in living standard, but it's not a failure.
If a retiree does not have to live under a bridge and merely has to cut back discretionary spending, I would not call it a failure. FIRECalc on the other hand would say that it's a failure. Conversely, if you follow FIRECalc or some retirement calculators to the letter to plan your spending, 100% success rate does not really mean 100%, unless the future is going to be exactly like some part of the past.
The future is still unknown, and the past is used as a guide because we have nothing better to work with. Planning makes it more likely that we can maintain our living standard, but it cannot make anything certain. And since I am not so rigid to demand the same living standard no matter what, I do not really worry about 100% or 95% success rates, as there are other unknowns that are more important. I still like to spend less in order to see my stash grow, but it is because I love counting money.
