The effective tax rate is just the tax divided by the taxable income.
The marginal tax rate is the percentage of tax applied to each additional dollar of income within a specific bracket, rather than being applied to the whole income.
Note that the marginal rate calculated can be influenced by the portion of capital gains that comprise the total tax.
The key here is that not all of the taxable income is being taxed per the tax table since the realized capital gains are removed and taxed at a lower rate (at 0%, 15% or 20%). So the marginal rate MaxiFi shows is the highest tax bracket in play when calculating the tax on the non-capital gains income. For reference, this follows the flow of the "Qualified Dividends and Capital Gain Tax Worksheet" included in the form 1040 instructions.