As you stated , since less taxes were withheld from your husband's paychecks, you are now being shown that you owe taxes. It would be a good idea to check the paystubs regularly so see how much is being withheld since most pay stubs not only show how much was withheld from that paycheck but it also shows how much has been withheld for the year.
Each company may have their own rules but it is possible that since the HR Department does not have a full picture of how much the total combined incomes would have been, they may have assumed that the total combined income would have been $32,000 and based on that they felt $500 was enough from a withholding perspective.
It would be a good idea to discuss this with the payroll department to be sure enough is being withheld.
