One possible explanation (pure conjecture) is that this is a consequence of the still prevalent sexist/old-boys-club mentality. Only the women who are so exceptional that there can be no question about their capabilities are the ones who end up on top. And the women who are equally qualified as the typical male CEO simply don't make it through the gauntlet.
This is an effect that minorities face in general. They have to be the absolute best in order to break through the glass ceiling. There are plenty of male CEOs who are utterly mediocre, and mediocre women never get a shot at the CEO spot.
Or, female CEO's are a more risky prospect than male CEO's and companies have different risk/reward motivations based on their situations. If a company's facing a tough grind for a few years they may be risk averse and stick with "safe bets" wherever they can.
Or, years of fighting "the patriarchy" has left females uniquely qualified to lead companies in the fight against changing market conditions.
Or, prospective female executives are probably very aware of the social and political climate we live in and that may make them more risk averse in regards to accepting positions, they may place a higher value on taking positions at companies on the upswing.
We can do this all day but I doubt the data's going to give a clear answer, we'll all see what we want to.
A much more likely explanation is selection bias. There would never have been a post like this if the result would have the other way around. Statistical variation makes it easy to find a starting point to show what you want.
I like your theory. Whenever there's talk of some sort of affirmative action, some people say "Imagine getting your job just because you're a woman". Well with this theory, there are men getting CEO job partially because they are men, even if they aren't very good.
This is an effect that minorities face in general. They have to be the absolute best in order to break through the glass ceiling. There are plenty of male CEOs who are utterly mediocre, and mediocre women never get a shot at the CEO spot.