Your percentages are different because cFIREsim isn't really set up to deal with the accumulation phase. There's no setting where you can say, "I want to FIRE as soon as my portfolio reaches $X, then run for 30 years to see if it'll last in retirement." I don't think comparing the 40% chance of success based on accumulation time to the success rates of a portfolio to go the distance in retirement holds water. It's bad math.
Naturally you're more likely to retire on an up year if you're plan is to hit a number and then quit working. There's also a whole bunch of
data that shows the sequence of returns in the early years of retirement are the most important. It goes without saying that if your returns are 0% over the first 10 years of retirement that you are in a much riskier position than the person who averaged a 6% return during those same years. Someone who is a little gun shy may work beyond their number, but remember that when cFIREsim tells you you have a 95% chance of success it has already factored in all the times in history where a retirement is immediately proceeded by bad returns. And in a few of those cases the portfolio fails, as evidenced by the percentage not being 100%.
What cFIREsim doesn't take into effect is our ability to use our brains. On threads like these I see so many people who make comments that allude to the idea that they want some type of method for their spending in retirement that will automatically account for that risk of a downturn after retiring. I know there are a lot of engineers here so I understand (I am one). But no method is required, just thinking. Since we know there is a very strong correlation between poor initial returns and portfolio failure, it's as simple as being proactive if you find yourself in this position.
For example, I FIRE'd in June. Many people would say they're expecting a market downturn soon. Let's say 5 years from now the average return for those 5 years was 0%. Based on sequence of returns risk, I already know that my portfolio now has a substantially higher risk of failure over 30 years, assuming I spent all the money I had planned to during those 5 years. If I didn't curtail my spending at all, I would consider picking up a part-time job to give my portfolio an assist. I'm only spending $40,000 a year from my initial $1 million stash, so a few years of 10-20k income will give me a boost. The next few years' returns would likely dictate how long I kept the part-time job. I could also lower my spending a bit if I didn't want to go back to work. Our bare bones expenses are $25,000 so if I'm really conscious of the sequence of returns risk maybe I just cut my spending hard in the year where the returns are particularly bad. No set method required, just a little brain power and flexibility.