The employers can argue that the employees in question work in different establishments.
Different Kind of Work.
They can claim the work is not equal in nature, quality, quantity or character; and therefore the pay need not be the same for different kinds of work.
The employer can show that the effort levels required are not equal.
They can argue that the level of responsibility differs between the jobs
The jobs are conducted under different working conditions.
Different Production Levels.
The employer can say that it measures earnings by quantity of production, and the quantity varies between the parties in question.
The employer can say that it measures earnings by quality of production, and the quality varies between the parties in question.
Finally, the employer can claim that the wage differential is based on additional factors (such as education, leadership capacity, or publishing experience.