The tax table is not actually a straight percentage. It moves in $50 increments, so the $12 tax jump was likely a result of being moved into the next $50 increment. It will probably make the most sense if you take a look at the link below and the tax table for your income level. You will see $12 increases in tax due for every $50 of income at certain income levels/filing statuses.
https://www.irs.gov/pub/irs-pdf/i1040tt.pdf
