The IRS has been banging this drum
loudly and often, but it's not clear why.
E.g., from the linked article
Specifically, married taxpayers who itemize deductions, with two children under age 17, income exceeding $180,000 from one or more jobs and who have $20,000 or more in nonwage income (dividends, interest or capital gains) are highly likely to have to pay additional tax when they file next year.
Using $200K W-2 income and $20K interest income, a $500K 4% mortgage, and $5K in property tax for the example couple living in California, the 2017 federal tax would have been ~$32,957.
For the same situation in 2018, the federal tax will be ~$30,227. The $4K Child Tax Credit more than makes up for the limit on state and local taxes for itemizing.
If the W-2 earner has "married with 3 allowances" on the W-4, there should be ~$600 refunded when filing.