Hypothetical possibility: members of group X are not perceived as 100% as effective as group Y because of pervasive bias by the employers that assumes their incompetence. They are generally perceived to be 80% as effective as a standard Y member despite actual 100% performance, and paid accordingly. A member of X needs to be 120% as effective as a Y member to be perceived at 100% Y efficiency because of stereotypes coloring their perception and an inability to objectively evaluate their performance.
Some non-hypothetical studies touching on this:
http://www.nber.org/papers/w9873.pdf http://www.pnas.org/content/109/41/16474.full.pdf+html http://advance.cornell.edu/documents/ImpactofGender.pdf http://www.socialjudgments.com/docs/Uhlmann%20and%20Cohen%20...