Paulines social security benefit of $15,588 was not included on her total income why not?

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Social Security Benefits are not always taxable.  If that was her only income and she is NOT married filing jointly, she does not need to file a tax return.

If she is single and her other income plus 1/2 of her benefits were less than $25,000, then her SS would not be taxable, therefore it would not be included in her taxable income either. 

https://www.ssa.gov/planners/taxes.html