TurboTax results extremely different than tax bracket calculations

Greetings,

My wife and I are filing jointly, but I did not make any income. I finished school and am writing a book, so no income from me. 

Her taxable income came out to $50,571, but TT says we owe $8630, federally. 

However, according to the tax bracket site: 

If taxable income is over:but not over:the tax is:
$20,550                                $83,550               $2,055 plus 12% of the amount over $20,550

 

My math is: $50,571-$20,550=$30,021, then multiply by 0.12 = $3,602.52. Add $2055=$5,657.52

That's a difference of $8,630 (what TT says I owe) - $5,657.52 (tax bracket calculations) =$2,972.48 difference NOT in my favor. 

We don't really have investments, or income from any other source. I don't know what I am missing!

Thanks for any assistance!

-Colin