Why does each charitable contribution reduce amount owed by a different percentage?

[pogam]

As I add each charitable contribution, I watch my amount of taxes owed going down but for each entry the amount that the taxes are lowered is a different percentage lowered.  It is about 24% on the average but I've seen a range from 23.8% to 24.56%.  Why the difference?

[SteamTrain · Accepted solution]

Because..the tax tables only work in $50 increments.  You could make a $1 contribution and get a $12 tax change if you were right at the next $50 block.  (not always $12...but you get the idea). then make another $40 contribution with no tax change

So you are in the 24% bracket, but where your income ends up  in the tax table after the contribution is a bit fudgy,.