My withholding in 2021 was commensurate with what I earned. But I just realized what the problem was by doing this exercise. I neglected to update something on my W2 when I retired - it slipped by me because I did actually retire and then went back part time. Thank you - that explains the difference - I would not have thought that it would have caused such a huge hit since the actual withholding shortfall would amount to maybe $2000 - anyway that is what it looks like in the pay stubs from before I made the correction and after.
