It’s not about equal pay, it’s about not paying differently simply due to gender. You can do so for other reasons even ones correlated with gender. It’s very hard afaik to prove such a case so a company needs to be really blatant about it.
California law is more strict. There is a whitelist of allowable reasons, consisting of: seniority, "a merit system", quantity and quality of producrion, and the catch all
> (D) A bona fide factor other than sex, such as education, training, or experience. This factor shall apply only if the employer demonstrates that the factor is not based on or derived from a sex-based differential in compensation, is job related with respect to the position in question, and is consistent with a business necessity. For purposes of this subparagraph, “business necessity” means an overriding legitimate business purpose such that the factor relied upon effectively fulfills the business purpose it is supposed to serve. This defense shall not apply if the employee demonstrates that an alternative business practice exists that would serve the same business purpose without producing the wage differential.
In particular, prior salary is explicitly not a valid defense.
By this standard, if there is a pay gap in California, the burden is on the employer to demonstrate they are not discriminating
