I suspect that your reported income is high enough to make 85% of your Social Security benefits taxable income on your Federal 1040 tax return.
You should be able to confirm that calculation by looking at box 6a and 6a on the Federal 1040 tax return. View the entries at Tax Tools / Tools / View Tax Summary / Preview my 1040.
Only the 85% taxable portion of your Social Security flows to your adjusted gross income and taxable income and Colorado is picking up the Federal taxable income to begin the Colorado state income tax return.
Since Colorado starts with its computation with only the 85% portion, only the 85% portion is deducted to accomplish 'fully deduct(ing) Social Security benefits from the state income".
@Mikes9
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