Sigh... No it's not, it is just comparing apples to oranges. If you cite the 20% number I know you have no interest in understanding the issue, because that number is a comparison across all jobs, and it ignores education, part time, everything. You don't have women and men working in the same company with 20% difference in wages (at least not significantly many - of course such things happen, but also with men vs men).
The 20% number definitely is not from the tech industry either.
So you have women picking different jobs. You have different incentives and preferences (motherhood is the big one here), different preferred industry (like media vs mining or whatever).
You also have women getting half of the income of their husbands, having the option to drop out of unpleasant jobs, not being under the same pressure to feed their family, and so on.
There are many many aspects to this. So far I haven't seen anything that convinces me women are being treated unfairly.
There are issues, but only if you consider staying at home with kids degrading. For example I suppose the incentive to get a good education is less if you know you will miss out several years where you could earn back the money invested. I don't think "unfair" is the right way to characterize that issue, though.
