What to do if there is an IRS discrepancy in amounts owed?

Why is there ordinarily a discrepancy between what the taxpayer calculates they are able to pay as opposed to what the IRS determines they can pay?

A taxpayer will provide their net income and indicate their actual expenses on the Collection Information Statement.  However, the IRS will use a taxpayer’s gross income and adjust a taxpayer’s actual expenses to an amount the IRS considers to be an “allowable expense”.