Client filed 2020 1040 MFJ.  He and his spouse received a Balance Due notice from the IRS dated in December 2021.  One notice was addressed to him, and a different notice was addressed to her. Everything, including the amount due is the same.

Why were there separate notices?  Does that fact that he is not a U.S. Citizen while she is a U.S. citizen explain it?

The month before, the IRS sent one notice addressed to both of them.

I don't deal with non-citizens much so there is a gap in my knowledge.