I think it's something that we impose upon ourselves. Imposter syndrome is very much present. You think you ought to work harder to get your promotion and you feel you have to delay it until you're really, really good, whereas men will normally ask for a promotion whenever they feel they deserve it. There's a very big difference in the choice and what women decide to do.
They want to make sure that, when they apply for a promotion, they're going to get it for sure. It's a different strategy.