It's simply the result of adding an amount of income to AGI and being able to subtract it back out the same amount as an itemized deduction. Until itemized deductions reach the level of the standard deduction, additional itemized deductions don't subtract back out.
Be aware, though, that that my answer does not say that once your other itemized deductions reach the standard deduction that doing a taxable IRA distribution followed by an equal deductible charitable contribution won't increase tax liability. It's entirely possible that there is a side effect of increasing AGI that is not offset by the deduction. For example, if you are married filing separately and made a Roth IRA contribution, the increase in AGI would cause your Roth IRA contribution to become subject to a 6% excess-contribution penalty unless the excess contribution was corrected. There are numerous other things that depend on AGI. The difference if you are over age 70½ is that money (up to $100,000) transferred to charity as a QCD would not add to AGI in the first place and the QCD would not induce any AGI-dependent side effects.
To really know how an IRA distribution and equal charitable contribution would affect your tax return you would have to simulate your entire tax return without the IRA distribution and charitable deduction, then add these two and see the net effect on tax liability. Also, don't forget to check the effect on state taxes.
