I've always understood that ss pay a larger proportion of earnings back to lower income levels, but I didn't think that it evened that out for a number of years with 0 earnings.
The way your SS benefit is calculated is that they sum up your 35 highest earning years (adjusting for wage inflation) and divide by 420 (the number of months in 35 years) to calculate your average monthly earnings.
A person who makes $100,000 for a few years and zero most of the rest* has the same average monthly earnings as someone pulling in only $12,000/year for all 35 years. And the person who earned a high salary for a few years and then FIREd benefits from the same tweaks to the benefits formula to make sure that person who only ever earned $12k/year isn't starving on the street in retirement.**
*Would have to have earn slightly above zero in a few more years in order to have enough credits to be eligible for Social Security, but even working a very part time job in college can get one almost halfway to being social security eligible.
**People can argue back and forth about whether this is a good thing or a bad thing, but it is how the system works whether we approve or disapprove.