Really? You need me to explain this to you?
OK, if there is no bias then everyone has an equal chance of progression - depending purely on their individual talents and qualities. If there is a skew towards one demographic as you rise through the ranks, that either shows that that demographic is inherently better suited to more senior roles or there is a bias.
If you don't believe there is a bias, what is it about men that makes them more suited to being CEOs? Or bankers? Or sewage workers?
Still waiting...
By the way, you missed out on one other factor - choice. People don't "choose" to become a CEO. They choose to aim to become a CEO, which requires a lot of hard work, sacrifice, selfishness and aptitude. Studies show that females in general are less selfish than men when it come to career matters. Plus of course many women choose to break their career to have children; it's harsh, but naturally any career break of that magnitude will negatively impact on a career path. Having made that choice, many people fall short of their aim to be a CEO. That's men and women. It's not because of their gender, it's a simple statistic.