The % success chances also all depend upon you never changing your spending levels, or getting more income in the future, even if you see significant economic downturns.
My "essential" budget (covering everything I need to have my home/car/food/utilities/internet/TV/clothes/etc) comes out to ~$28k/year. So I could theoretically cut down to that level of spending if I eliminated all my "outside the house fun/enjoyment" money from my budget. I could still garden, go for walks, attend free events, watch TV/movies, play games, read books, etc all for that amount.
My "overall budget", however, is planned for needing $60k/year instead.
So while an "80% chance of success" on a $60k/year planned spending (using constant spending forever) may seem a bit low, it doesn't factor in that I will probably do some consulting work for a few months a year when I first "retire", or that if I watch the stock market drop I'll probably not spend $15k on vacations that year or I'll cut back on buying "stuff" that I probably don't need etc during such times.
Unless you're one of the "Mr Money Mustache" follower types that is planning on retiring on a 4% SWR that barely covers a "minimalist spending" lifestyle, I'd say that a 95% success rate, when adjusted for how most people would actually behave if things went poorly for a while, is probably more like a "you'll be absolutely fine unless something close to world-ending or other similar catastrophic scenario occurs". I'd say for most FIRE people, 80% is probably plenty, and 90% is practically a guarantee of success without a future calamity that would probably screw over the 95% success rate plan as well.