I'm not really convinced it's a discrimination thing. I think it's women not asking for raises or higher salaries when they start a job. When you're negotiating a job offer, they have to low ball you, but there's usually a range they can offer, and if you just ask they will give you more. Some just don't ask. Women have to deal with a lot of issues like it being ingrained them since elementary school to not seem bossy, so they learn to not speak up and be assertive. Impostor syndrome is also pretty common. I was pretty shy, unassertive, and underpaid my first year in software engineering. I was lucky to work some place where I was encouraged to speak up. After my confidence was built up after a year, I asked for a raise from $15/hr to $25/hr (I was part time as I was finishing up school.) My boss had to ask a higher up, and they gave me $28/hr. All I had to do was ask. I'm sure that at some companies some women might never break out of their shell though, and that's unfortunate, but it's really hard to undo years of conditioning.
I guess my point is it's not really an issue of people at companies being discriminatory, it's how our society raises our girls. As an aside, I think that's also why there isn't many women in software engineering, because they aren't exposed to programming as children.