On December 31, 2012, the IRS issued tax withholding tables on the assumption that we were going over the fiscal cliff.
Technically, we did go over the cliff, but only for a few hours. In addition, the fiscal cliff deal that was passed on January 1, 2013, made the tax changes retroactive to the beginning of the year (in this case, just a matter of hours). So, before we hit the ground after going over the cliff, the Congress hit the rewind button and we ended up on the edge of the precipice, walking backward away from it.
Tax rates have changed for everyone in 2013, more for some than for others, though. Still, the income tax withholding tables the IRS put out on December 31 had to be revised after the fiscal cliff deal was reached and signed into law. Yesterday, the IRS, in an admirable job of getting guidance out quickly, issued revised income tax withholding tables in its revised Notice 1036.
You can read about the revised Notice 1036 and get the revised income tax withholding tables at this link: Revised Notice 1036.