Is there an explanation as to why turbo tax considers only about half of my social security retirement income as being taxable on the federal level?

[DoninGA]

Up to 85% of Social Security Retirement/Disability/Survivors benefits becomes taxable when all your other income plus 1/2 your social security reaches:

Married Filing Jointly - $32,000
Single or Head of Household - $25,000
Married Filing Separately - 0

The Social Security Benefits Worksheet will show how the taxable amount of the SS benefits were calculated.

If using the TurboTax desktop editions, click on Forms

In Forms mode click on Social Sec on the left side of the program screen to open the Worksheet.

If using the TurboTax online editions - 

Click on Tax Tools on the left side of the online program screen.  Then click on Print Center.  Then click on Print, save or preview this year's return.  Choose the option Include government and TurboTax worksheets