That's because there is no discernible wage gap. There is an earnings differential that has been pretty much fully explained.

For example here, fairly comprehensively: "both men and women unconsciously make trade-offs that affect how much they earn. Farrell clearly defines the 25 different workplace choices that affect women's and men's incomes -- including putting in more hours at work, taking riskier jobs or more hazardous assignments, being willing to change location, and training for technical jobs that involve less people contact -- and provides readers with specific, research-supported ways for women to earn higher pay."

https://www.amazon.com/Why-Men-Earn-More-Startling/dp/154275...

But there are many other sources. One example was a study of wages at the Massachusetts Bay Transportation Authority, which has completely codified/rigid pay structure and still managed to "discriminate" against women, pretty much at the same rate as the national "gender pay gap". How? The men did more overtime, more shifts at inconvenient times etc.

In fact, given all the evidence, as far as I can tell it is overexplained.

Alas, this doesn't fit The Narrative™.

(There are entire books about this! http://amazon.com/Why-Men-Earn-More-Startling/dp/0814472109)

So brave!

Way to go, California!

that bias-based unequal pay for women is largely a myth, and that women are most often paid less than men not because they are discriminated against, but because they have made lifestyle choices that affect their ability to earn.

http://www.amazon.com/Why-Men-Earn-More-Startling/dp/0814472...

The pay difference is a matter of a multitude of life choices made slightly differently between men and women.