Three numbers decide the credit
Revenue Procedure 2025-32 publishes an earned-income amount, a maximum credit, and phase-out thresholds that differ for joint filers. This page uses those amounts rather than the IRS $50 lookup tables, so a table cell can differ by a few dollars.
A joint return starts phasing out later. Married filing separately is treated like every other non-joint status; the narrow separated-spouse rule is not tested.