Why doesn’t the amount in Box 1 (Wages, Tips and Other Compensation) match the total gross earnings on my last payslip of the year?

The W-2 Box 1 amount is federal taxable wages equal gross earnings minus pre-tax deductions such as health/dental/vision insurance, retirement contributions, Tax-Deferred Savings Plans like 403(b) and 457 retirement accounts, and FSA Medical and Dependent Care.