I changed the years in my answer to be one year earlier.
Presumably you had other distributions or Roth conversions from your own traditional IRAs in 2018 but your wife did not, resulting in line 4 of your Form 8606 being populated but line 4 of your wife's Form 8606 being blank. If this presumption is wrong, you either reported a traditional IRA distribution or Roth conversion from your own traditional IRAs that you did not actually make or you failed to enter a traditional IRA distribution or Roth conversion from your wife's traditional IRAs that she did make.
(Note that distributions or Roth rollovers from a 401(k) or similar qualified retirement plan are not reportable on Form 8606 since 401(k)s and similar qualified retirement plans are not IRAs.)
