The most likely answer is that you've entered qualified dividends and/or capital gains on your tax return.
When these types of income are reported, taxes will not be calculated using the tax tables, but using either the Schedule D Tax Worksheet provided in the Instructions for Schedule D, or the Qualified Dividends and Capital Gains Tax Worksheet in the Instructions for Form 1040 and 1040-SR.
These worksheets will result in lower taxes than the tax table or tax schedule.
