Why is my taxable amount of social security being deducted from my federal income on my New York State Tax return?

Social security is never taxable in NY state, even though it can sometimes be taxable on your federal return depending on your other income. 

The Federal taxable portion of Social Security does not carry over to New York because all Social Security retirement benefits are exempt from NY taxation. Income from retirement accounts or private pensions is deductible for up to $20,000.