Why do feminists believe affirmative action is necessary?

The statistic that women make 77% of what men make is derived from averaging up the salaries of all the working women and averaging up the salaries of all the working men. When feminists promote this statistic, they make the umption that an equal number of men and an equal number of women work in the same job. But yet, they say that affirmative action is necessary because women are underrepresented in many of the high paying jobs. So, which is it?