Symphony orchestras used to claim to be the equivalent of a 'meritocracy' (don't think that they used that term, though). They selected strictly by audition, relying on the ears of the (male) conductor/director, and less than 10% of the musicians selected were women. When they changed to have the musicians perform behind a screen, and without identifying them by name, the number of women selected increased dramatically.
A similar dynamic happens in the tech world. The people determining 'merit' have that same built-in bias against females, and will judge their work as lower than it should be. The bias need not be conscious, and in most cases the men think that they are being fair and honest, but it's there.
This exact example features in the blog post draft I copied most of my comment from :-).
Even more pernicious than that though - you say "the men think they are being fair and honest", but research repeatedly shows that women are also less fair to other women. This is important to keep in mind because it reinforces the fact that most of the biases that work against objectively determining merit are part of the fabric of our society. They are biases that you will tend to hold even if they hurt you personally.
Many men bristle when they are confronted with this sort of research and say "I'm not a sexist" or "I'm not a racist". That's the wrong way to think about it. We live in a deeply unfair society, and unless we constantly, mindfully, consciously work against that unfairness all the time, we will fall victim to it ourselves and perpetuate it.
A similar dynamic happens in the tech world. The people determining 'merit' have that same built-in bias against females, and will judge their work as lower than it should be. The bias need not be conscious, and in most cases the men think that they are being fair and honest, but it's there.