Why would Social Security benefit line 20b increase when I only amended my FED. to increase line 15b (IRA distribution)?

[jeeturbo2017]

Line 15b was increase by 2,139, which is an additional IRS Distribution that was omitted the first time filing.

I prepared an  amended the Fed line 15b for the gross of the distribution.  

Line 20a ( Social Security benefit did not change, BUT 20b increased by 1,818..

[Lisa995]

15a is the gross, 15b is the taxable distribution, if 15b has additional income, that  makes the taxable amount of social security increase as well.

[dmertz · Accepted solution]

The amount of your Social Security income that is taxable depends on the amount of your other income.  $1,818 equals 85% of $2,139 and is exactly the amount that the taxable amount of your Social Security income would be expected to increase.