Because Firecalc shows a higher probability of success if we take it at 62 than other years.
FWIW, was running Firecalc earlier this evening. DH is already on SS, but I ran it for me starting at 62 (2017) or at 66 (2021). I input spending numbers and other numbers. In our case, we will have very high withdrawals for the next couple of years (about 15% of the portfolio each year) mostly for child college reasons. So I do expect the overall portfolio to lessen between now and 2017.
Anyway, I ran this with a couple of different starting portfolios - one is the actual portfolio we have now and the other was the portfolio we had at the start of this year. Both times I ran it saying 2013 for retirement year (DH is already retired).
Anyway - When I ran it with the current portfolio amount it was slightly higher percentage (98%) to take SS at 62 than at 66 (97%).
However, when I ran it with the portfolio amount we had at the start of the year which is about 95% of the current portfolio then it came out a slightly higher percentage to take SS at 66 rather than at 62.
These were all based upon 30 year retirements - DH is currently 65 and I am currently 59.
FWIW, I also ran everything for a 36 year retirement. Changing it to a 36 year retirement didn't make any difference in terms of whether taking SS early or late would make a difference.
Warning: Firecalc gives surprising answers depending on your current age.
If I'm 55 and using Firecalc to plan ahead, it may well tell me that I can spend slightly more if I start SS at 62 rather than waiting till 66.
However, if I'm 62 and using Firecalc to decide what to do today, it will probably tell me to defer till 66.
Remember that we expect very high probabilities of success when we run Firecalc. It may run 111 start years, but 100 of them are basically irrelevant for this type of question - you're going to succeed with either strategy. The differences boil down to a few start years. Very subtle differences in orders of returns in a very few years, interacting with the options, can make a difference.
If you're under 62, I assume you'll be running this again before you actually pull the trigger on SS. Don't be surprised if the Firecalc recommendation changes.
Here are some details that should be easy to check:
I tried a "standard" Firecalc run (all the default assumptions) except that I added SS. It was either $15k starting immediately or $20k starting in 4 years. This would be a typical at-age-62 question. Firecalc solved for safe withdrawals of $44,813 and $46,663 respectively. A clear win for deferring.
To double check that, I input exactly $45,700 of withdrawals, and got 8 failures for starting immediately vs. 5 failures for deferring. Both strategies failed for start years 1965, 66, 68, 69, and 73. But the early start also failed for 1964, 66, and 1906. All of these years have low returns in the "early" years of the projection, meaning the early SS payments don't earn any significant interest.
Then I input either $15k starting in 7 years, or $20k starting in 11. This would be the input for someone who is currently age 55 and thinking ahead. Firecalc solved for safe withdrawals of $40,774 and $39,950 respectively, a narrower victory for starting at 62.
When I checked that by inputting $40,000 of withdrawals, I got 5 failures for starting earlier and 6 failures for starting later. Again, both failed for starts in 1965, 66, 68, 69, and 73. This time the extra failure was the 1972 start year.
Looking at the results, I think it would take a lot of effort to determine exactly how the pattern of returns in those few years impacted the results. I'll just say that these differences are based on very little data.