This is due to how Adjustment of the Reduction Factor (ARF) and the Earnings Test are applied.

If excess earnings were charged against reduced early retirement, spousal, or widow(er)'s benefits, then the original reduction factor is adjusted to exclude the months that benefits were not received due to the earnings test.

Partial months of benefit deductions (i.e., months for which you don't earn enough to lose all that month's benefits) due to the Earnings Test are treated as whole months when the Adjustment of the Reduction Factor (ARF) is applied at full retirement age. Because of this quirk in how the ARF is applied, the lifetime benefits for this filing strategy can be higher than if you file and suspend at your full retirement age or simply wait until age 70 to file for retirement benefits. You lose a portion of a month’s benefit due to the Earnings Test, but the ARF treats the deduction as if 100% of a month’s benefit had been lost.

Essentially, you can receive a free partial month benefit with no permanent reduction in your benefit!

There is an option on the Social Security application, Form SSA-1, to declare that you "want benefits beginning with the earliest possible month providing there is no permanent reduction in my ongoing monthly benefit." Since you don't know your future income with certainty, checking this option ensures that your benefits won't be permanently reduced.

For example, you could file for your retirement benefit a month before FRA and then lose your job the next day. In this case, you won't earn enough money to lose even a dollar of benefits under the Earnings Test. And if you don't check off this option, your retirement benefit would then be permanently reduced based on your having taken it one month early. That would be a bad thing as you would not have that reduction undone via a full month's crediting under the Adjustment of the Reduction Factor.

Learn more about file and suspend. 

What is filing and suspending?

How do I model filing and suspending?