It is a negotiating problem, and I think that that is an issue (be it gender-related or not). It rewards competitive thinking over cooperative thinking - people who'd rather work in a fair and equal environment are more likely to accept the same salary they perceive others will get - i.e. usually the proposed salary. Competitive minds have less qualms about claiming higher rewards.

Mechanisms like publishing salaries allows for a fairer salary whether you are cooperative or competitive. If you earn more, you probably can justify it in front of colleagues and bosses.

Some say cooperation over competition is a 'female' trait, but personally, I feel the same. I am successful at negotiation, but I'd rather know that my colleagues earn the same when they work the same, which is why I try to encourage talk about salaries.