Even if the 80% thing is true, this is not a difference to lose the sleep about. It should be fixed, but the statistical noise is too significant to aim at 100%. The marginal value of money is log(x), and log(0.8) is almost 0.
When women were limited to a couple professions (nun, teacher, nurse) it was a buyer's market, and even brilliant women would have to give in to minimal salaries, or opt out of professional work. At that time, the difference was like 10x, so women made 10% of a man, and log(0.1) is a significant number. This was successfully resolved (with the help of two world wars and the invention of washing machine).
When women were limited to a couple professions (nun, teacher, nurse) it was a buyer's market, and even brilliant women would have to give in to minimal salaries, or opt out of professional work. At that time, the difference was like 10x, so women made 10% of a man, and log(0.1) is a significant number. This was successfully resolved (with the help of two world wars and the invention of washing machine).