In my case, the amount on Form 1040 line 25c was "Additional Medicare Tax withholding on Medicare wages". It came from line 22 of Form 8959 under "Part V Withholding Reconciliation". It looks like my employer withheld an overly-large amount on Form W-2 box 6 (shown on line 19 of Form 8959). An amount on line 20 of Form 8959 was multiplied by 1.45% to get the "regular Medicare tax withholding on Medicare wages", and then line 21 of Form 8959 is subtracted from line 19 of Form 8959 to get the final line 22 value on Form 8959. Again, in my case, line 22 of Form 8959 is the data source for Form 1040 line 25c, which is "Federal income tax withheld from Other forms".
