There are many theories behind it, none of them either proven or debunked. One of them being men being taken seriously more, so they'd rather have a man than a woman, so let's pay more to men to lure them in... Sexist reason, yet (unfortunately) plausible.
Another theory seems to be that men are better bluffers than women, and that women are too honest at times. Not sure, but a possibility.
And another one is also that men are the longer they work somewhere more willing to negotiate with the boss about a pay raise than women, and men therefore also willing to brag more to the boss to the importance they've been to the company. This one can be combined with the buff theory...
There are a lot more theories, I've heard about this matter. Some theories really blaming the women themselves for not standing up for themselves like males do, and others blaming the employers for sexism. In the end it's likely a combination of factors. Please note, it's only something of the last few decades that women are getting more equal to men than inferior to men. My own bother quit working because I was born, like women were expected to do back then. And I'm not even that old, and my mother is even still alive. It would not be realistic to think that all the traces of the old situation would be gone by now.
Of course, favoring women over men, is as much sexist as the other way around. Although the difference between man and woman being more than just men having something between their legs women have not (scientific fact), I've never understood why that should be reason to see them as inferiors. But for the past thousand of years this was the public view. My own grandmother, a woman herself, for crying out loud, was heavily against the emancipation of women. She really held tight to the old man works for money and woman does the cleaning, cooking and taking care of kids, role pattern.
I don't think this game will make much of a difference (it's because we're in August now, but I'd almost take it for an April fools, joke), but time is needed, and we all need to accept the idea of woman and man being 100% equal.. Still different, but still equal... Much has been achieved already, but we're simply not there yet.