This is how the tax tables work.
The Federal tax tables give the tax by tranches of $50 of taxable income. The tax will be the same within the same tranche of $50. As you increase the donation number, at a certain point, the taxable income will move to the next lower tranche and the tax will decrease and the refund will increase by $6 as you are in the 12% tax bracket ($50 x 12% = $6).
The same principle applies to state tax. The fact that state tax did not go the lower tranche at the same point is because taxable income is different between and state.
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