Amended return and deductions

So, I am thinking that adding a deduction you would either add what was omitted or use a new total for the deductions? I had 2 amounts for a retirement fund not thru an employer. Will the IRS know if I added a new total or just the amount that was not reported on the original filing? I thought you would add what was not on the original filing but instead I added a total. Which is the correct way?