They key point here is that workers are not identical. If you are getting paid less than your colleagues, that could be because you got a raw deal, but it could also be because you aren't as good as them.
Even within a job title, there are good reasons why pay should not be the same. For example, the job ladder represents both authority and technical ability, and these sometimes need to be decoupled. A very talented individual might have technical ability that deserves high pay, but not the industry experience to hold a position of leadership or authority. So they get hired at a lower job title, but with higher pay than their peers.
Even within a job title, there are good reasons why pay should not be the same. For example, the job ladder represents both authority and technical ability, and these sometimes need to be decoupled. A very talented individual might have technical ability that deserves high pay, but not the industry experience to hold a position of leadership or authority. So they get hired at a lower job title, but with higher pay than their peers.