Yes, capital gains and how they are taxed is the primary explanation for what you are experiencing.
A side by side comparison of the 2022 "Schedule D Tax Worksheet" and the 2023 "Qualified Dividends and Capital Gains Tax Worksheet" should explain the difference.
The bottom lines, on each worksheet, has a comparison of the actual tax (with capital gains rates) vs. the "coulda been" tax from the tax table.
