Two numbers doing different jobs
The Additional Medicare surtax is 0.9% on wages above a threshold. What makes it awkward is that there are two thresholds, and they are not the same.
Section 3102(f) tells your employer to start withholding once it has paid you a flat amount, regardless of how you file. Section 3101(b)(2) sets what you are actually liable for, and that figure depends on your filing status. The employer number is a withholding instruction; the employee number is the tax.
Where they come apart
Three situations, and none of them involves anybody making a mistake:
Two employers. Each withholds only on the wages it pays, because neither can see the other. Two jobs each below the flat threshold produce no withholding at all, while your combined wages may be well above your own threshold.
Filing separately. That threshold is the lowest of the three, at half the joint figure rather than the same as single. Somebody filing separately can owe the surtax on wages far below the point at which any employer begins withholding it.
A joint return. The couple’s threshold is shared and is less than double the single one, which is the subject of its own guide.
It is not indexed
The thresholds were fixed in statute in 2013 and are not adjusted for inflation. Every year of wage growth brings more people above them, which is the mechanism working as written rather than an oversight. A salary that was comfortably below the line a decade ago may not be now, without anything about the rules having changed.
Our surtax calculator shows both thresholds, what your employer withheld against its one, and what you are liable for against yours.