Why doesn’t the amount in Box 3 (Social Security Wages) match the total Gross Earnings on my last payslip of the year?

The W-2 Box 3 amount is taxable wages subject to Social Security taxes. This differs from your Gross Earnings because there are certain deductions that reduce your Social Security taxable wages, such as the pre-tax medical/dental and vision insurance, and FSA Medical and Dependent Care. Retirement deposits and Tax-Deferred Savings Plans do not reduce your Social Security taxable wages. There is also a cap on Social Security taxable wages each year employees with earnings in excess of this cap will no longer have a deduction for Social Security tax for the remainder of that year