Standard v. Itemized

I am preparing an amended return for other reasons for 2017.

The original tax return prepared by another accounting firm claimed the itemized deduction instead of the standard, even though the standard was higher. I can't for the life of me figure out why the software (looks the same as mine) used itemized instead of the higher standard deduction.

The person was not a dependent, and was not married, filing status was single. Why would itemized have been claimed if it was lower?

Does the state have a lower standard deduction than Federal?

Possible explanations: (1) taxpayer is MFS and agreed to itemize as it as beneficial to the spouse, (2) as noted by Webster, the state might require conformity with the federal return, and itemizing for both federal and state produces a holistically lower tax liability than using the standard deduction for both federal and state, (3) error by the prior preparer.

There is a nuance with the calculation of itemized deductions in 2018 where the standard deduction, while appearing to be larger, actually increases the taxpayer's tax liability vs. claiming the itemized deductions. It had to do with the AMT threshold...basically, the benefit of itemized deductions (while lower than the standard) kept them out of AMT, but with the standard, it would have kicked them into AMT. The number startled me at the time, it was a difference of about $7k and appeared wrong at first...but low and behold, the software got it right.

Maybe the original tax preparer accidently checked the box "force itemized deduction"

Itemizing may have given taxpayer a refund on state taxes making the state refund taxable the next year. Which taxpayer/tax preparer was trying to avoid for some reason including that taxpayer is tired of keeping tracks of itemized deductions for what becomes a very small difference for the effort.

Maybe the original tax preparer accidently checked the box "force itemized deduction"

I think that is the most probable reason too.

Similar situation actually happened to me years before. Client dropped in after the tax season and we talked. Discussion gradually drifted to a scenario that he had itemized instead of claiming standard deduction in that year's tax return. So I checked the "force itemized deduction' box in the tax program in order to give him the result. But then I forgot to uncheck the box after the meeting. Fast forward to next tax season, the tax information of this tax client was updated from the previous year. I guess you know what had happened by now already...the 'force itemized deduction' box checked in the previous year tax data file was carried over to the current year. As a result, itemized deduction was claimed. I caught that 'would-be error' only during the final walk-through of the tax return. What a close call.

My opinion is check-box information like that should not be carried over to the following year's tax return.

Mystery solved. Taxpayer is a PR resident and they must allocate a portion of their standard deduction or itemized deduction to their exempt income.