My total itemized deductions exceed the standard deduction, but my total federal/Washington, DC tax due is lower if I take the standard deduction. Why is this?

Not all deductions (such as Medical) count at 100%.  And there is a $10,000 cap on State/Local/Sales/Property taxes combined.  Charitable is limited to a percent of your income as well.

If you have Mortgage Interest, be sure to indicate that it is secured by the property.

See this re allowable DC deductions, which are different than Federal:

https://otr.cfo.dc.gov/page/individual-income-tax-filing-faqs