Why isn't my Traditional 401k contribution deductable, since neither my wife nor I was covered by an employer Retirement Plan.

It's complicated.  I terminated my employment on 12/31/17 and was therefore not covered by a Retirement Plan in 2018.  However, due to a stub payroll cycle, my ex-employer withheld and contributed one last 401k contribution in the first week of January 2018.  So, I was not covered, yet the stub payroll shows a 401k contribution which shows up on my W-2.  The fact is that I was not covered in a Retirement Plan in 2018 because I was not an employee.  My AGI well-exceeds the income limits if I am viewed to be under a plan for some reason.

What I would like to do is contribute the difference between the maximum allowed and the stub 401k payment, and have my wife contribute the maximum, and have the sum of the two contribution be fully deductible--given that I was not covered by a Plan and therefore should have not income test.