It's something Marisol Franco with the Women's Foundation California has often wondered herself.
"My parents were both cooks, working the same exact job. And I noticed that my mother was making less than my father," Franco said.
"And it was just the most obvious form of seeing the discrepancy of pay," Franco said. "They were doing the exact same work, in the same restaurant."
Franco said it's a systemic issue, in part because women tend to take more time off work than men to deal with family needs, such as the care of an elderly family member or the birth of a child.
"If they have children, they may leave the workforce," said Franco, who also said that every week they take off from work means their male counterparts doing the same job move further ahead, which can often cost them merit-based incentives.
"And so (the women) aren't going to get that annual merit raise because they were gone six weeks or even three months, to take care of a newborn," she said.
And Franco said the problem can quickly compound itself when a woman begins to search for a new job. Not only because there may be gaps in work history, but because their salary is possibly lower to begin with.
"Let's say the last thing you did, because you were in between work, you did part-time. And maybe you took that because it fit the schedule at the time," Franco said.
"Then you're going to be undercut."
Franco said there needs to be more transparency in the hiring process. Not only should employers be required to list salary ranges for all positions, but she's an advocate for people to share salary information with each other, even though that's often considered to be taboo.
And while Franco said workers should not be afraid to ask for more money, the power still rests with employers.
"There's a responsibility on the employer to make sure that they are offering the highest amount they can when someone from a historically underrepresented community is coming in, rather than feeling like we are saving money because this person only asked for this," Franco said.