A couple Odds&ends
1) If your taxable income is still below 100,000.....then the tax tables are used for at lest some of your income and those tables have tax jumps...not smooth calcs.
2) the additional income somehow affects some other deduction or credit. To see if that's the case, you'd need a printout of your 1040 both before & after you put in the box 3 $$.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
