You are correct there could be a big disparity. The total tax liability is all taxes including any excise taxes for over contributing to retirement or HSAs and self-employment taxes, etcetera; generally those items on schedule 2 (and the list is long). That total tax figure is more than income taxes. The total tax line also includes subtractions for nonrefundable credits not taxes the taxpayer actually paid.
The income tax liability is generally line 18 however for the purist it may be line 16 plus any AMT from Schedule 2 line 1.
As noted in the discussion a useful calculation for one may not be what another is looking for.
TurboTax, it appears uses the total tax line / adjusted gross income to arrive at the effective tax rate.
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