Legal Question in Employment Law in Washington

Assuming there is no collective bargaining agreement in place that says otherwise, and assuming the motive for the difference in salary is not based on an illegal reason (you've ruled out gender; other possibilities are age, race, ethnic origin, religion, sexual orientation or marital status) there is likely no violation here. The employer's reasons could be anything from difference in experience levels to a more restricted budget to no good reason at all, but as long as you are being paid at least minimum wage, the employer can set the rate of pay anywhere.