It's a real challenge to eliminate "differentiators".
For example lets say a man was hired by a company in 2001 and a woman was hired during the Great Recession when there were 10 applicants for every job opening. Chances are she would accept just about any salary the company offered. Where as in 2001 the company may have offered the same salary to the man but he told them it was below what he was looking for.
Two factors came into play one is the state of the economy at the time the hiring took place and the other was the man forced them negotiate or he'd walk away. Some people (hate negotiating) whether it be job offers or car buying! If someone is willing to take less the company will oblige them!
Oftentimes there are other differentiators. Two applicants have 4 year degrees. One went to Harvard and the other got his from University of Phoenix. Odds are the company will offer to pay the Harvard grad more. Not all degrees and schools are respected equally.
Relevant past experience is another differentiator. One person may have come aboard from a direct competitor with a proven track record and the other person has "related experience" in a (similar) industry.
Last but not least some people simply interview better than others. Two applicants may have similar qualifications according to their resumes. However one had "glowing references" and came off much sharper in the interview than the other applicant.
If the company has two positions to fill they'll likely hire both of them but offer one more money.
Other variables also come into play such as the nature of the job. One person may have a B.S. degree and is earning $40k per year making calls from their cubical while the other is a high school drop out chopping down trees in the forestry industry or on a ship doing the "Deadliest catch" while risking their lives every single day.
They'll probably earn more money!
The only way to create equal pay is to force all companies in their particular industry to adopt something similar to union levels for exact same positions. For example (all) "level 1" reps make $10/hr and after 90 days you will get a 75 cent raise and then outline the various raise steps according to job levels. If someone tells you what their level# is and how long they've been with the company you'd know how much they earn per hour.
Naturally you could never make sales commissioned jobs equal as one's income is based upon their performance. It's an eat what you kill world.:)