Why might two employees at a company earning the same gross pay have different net pay

The correct answer to this is because of their deductions.

Two employees may earn the same gross pay, but have different net pays. All employees complete a W-4 form for their payroll departments. On this form you indicate how you want federal taxes withheld from your paycheck. Chances are they each completed the form differently and are having a different level of federal taxes withheld.