When all other factors are accounted for, women tend to make anywhere from 93% to 95% of their male counterparts. What causes the 5% to 7% discrepancy? Is it just plain old sexism? In general, do men negotiate for higher pay, more hours, bigger raises, larger bonuses, etc... than women? I don't know.
The 72%, or 75%, or 77% or whatever number in the 70%-79% range that gets used all of the time compares the average pay for every man compared to the average pay for every woman. So every man who is a construction worker, mechanic, attorney, machinist, doctor, engineer, business owner (fields where men are the predominant work force) has their pay added up and averaged. And then every woman who is a secretary/assistant, teacher, server, cashier, store associate (fields where women are the predominant work force) has their pay added up and averaged. Guess which group is going to have the higher pay? The group with the work force whose jobs are traditionally better paying. That's just basic math.
I'm not saying that sexism doesn't exist. But if you have a business where 2 of your employees have the same educational background, the same number of years of service, work the same number of hours per week, take the same amount of sick leave, have the same job title and the same responsibilities, and you're paying one of them 75% of what you're paying the other one, then you're gonna have the world's easiest wage discrimination lawsuit on your hands, regardless of the sexes of the people involved. If the person getting paid less was a woman, then you're really going to have a bad time when the case goes before a judge.