As an example, EIC is calculated on both (1) earned income and (2) AGI. You then get the lesser of the two. In addition, it is calculated on a "bell curve." That is, it goes up as your income goes up to a certain point, but then starts to come down after.
Also, as your income goes up, you can move into a higher tax bracket. As for Unemployment Comp, typically there is either 10% withholding on it or none at all. But if your actual tax rate on your total income is more than 10%, it "eats away" (reduces) at the refund that was there before you entered the Unemployment Comp.
