There is a special circumstance where the taxable Social Security benefits is calculated on a different form than you might expect. 

If you are also contributing to a Traditional IRA, then your taxable Social Security benefits is calculated on the form titled IRA SS Wks.  You will see that title in the forms list when you go to Forms Mode using a CD/downloaded version of TurboTax.  That worksheet will show you the full calculation. 

There will also be an asterisk shown at the bottom of your SS Benefits Worksheet beside line 20 to indicate that the calculation is shown elsewhere.