Because some folks I know got hit with this one recently, here’s a reminder that when you are Married and you file your W4, the “married” checkbox makes the form assume you are the sole earner for your family, and it calculates your withholdings accordingly.
Yes, this is idiotic.
No, I don’t know why the W4 is written that way.
Here is a Personal Finance Reddit post with more information. As always, the IRS is the final arbiter on this stuff; internet people can try to explain it but always go back to irs.gov to be sure.
Emilybook 