The IRS response is not surprising. Although they will recognize the income was omitted from where it should have been on line 21, they will not recognize that it was reported on line 7 because it is not income that should be on line 7. Many things depend specifically on the amount on line 7, so, as Critter#2 said, other areas of your tax return can be affected by the change needed to the line 7 amount and the only way to be sure that your tax liability was correctly determined is to prepare an amended 2015 tax return. The result will be different from what the IRS claims to be you additional tax liability, but it might not be a wash either.
