Get your taxes done using TurboTax

It's the tax tables.  To make taxes easier for most, the IRS gives tables with the tax rather than using a mathematical formula.  As you can see in this example, there are situations where adding $1 of income can increase the tax by much more than $1.  A $12 increase would suggest your overall income is in the 24% bracket.

Then, gambling losses will not decrease your taxable income unless your total itemized deductions are more than your standard deduction.  Itemized deductions including state and local taxes, mortgage interest, and gifts to charity.  If you have not entered all your itemized deductions yet, your tax may change as you continue to add information.