It sounds like the real reason for inequality is that the pay for "janitorial work" has simply decreased. The article makes a point to say that the pay is the same, accounting for inflation, but it's disingenuous not to highlight total comp, which Ramos had significantly less of than Evans.
The reason for inequality is that people are not equal. The work they do produce is not equal in value. More people can do a janitor's job, and/or do without a janitor, so the value of a janitor's work is less. More supply, elastic demand, means lower price point. Through technology, some people are able to produce much more than others, so inequality is inevitable unless there is a balancing force (aka taxation).
Consider a fast food restaurant with one cleaner, two people at the tills, three people in the kitchen, and one manager. Remove any of them and the lines will become too long, the place will become unacceptably dirty, etc. However, none of the employees is "producing" more than any other.