Most likely the estimate provided by the SSA is based on expected future earnings.
Generally - they assume that your future earning will be the SAME as during the last year - and because your earning was reduced - correspondingly - that affects estimated.
Many people wonder how the SSA figures their Social Security retirement benefit.
The Social Security Administration bases Social Security benefits on your lifetime earnings.
They adjust or index your actual earnings to account for changes in average wages since the year the earnings were received.
Then the Social Security Administration calculates your average indexed monthly earnings during the 35 years in which you earned the most.
And after that - they apply a formula to these earnings and arrive at your basic benefit, or primary insurance amount.
This is how much you would receive at your full retirement age.
So depending on your earning record - it is possible that there are some missing years or earning.
Also additional years of earning might increase your benefits.
If so - that could affect your social security benefit calculations and you may contact the Social Security Administration and verify if all information is correct.
Following publication might be helpful for better understanding the calculations.
Let me know if that answered your question?