I had a similar situation: 12k in itemized deductions vs 24,400 the standard way.
I figured it out by completing the return, not filing, printing out both returns, and comparing the PDFs side by side.
In my case, the issue came down to differences in AMT calculations. While itemizing, only the state and local portion is added back to AMT income, while if i took the standard deduction, all of that is added back. Itemizing made my AMT income and tax lower.
This alone wasn't enough. The other issue is that I have a credit for prior year AMT I'm carrying forward. By lowering my AMT income, TurboTax was able to claim more of of my prior year carry forward as a credit in the current year.
In my case this turned out not to be a good deal: burning $5k in carried forward credit to get an extra $750 in refund is an 85% percent penalty to get my money a year early, which is just awful.
In short: in my case TurboTax is being incredibly short sighted, optimizing only for the current year return number, but would have cost me money in the long run.
I went with the standard deduction.
