There are several reasons; the American Association of University Women (AAUW) mentions gender bias, discrimination, job tenure, education level, work experience, hours worked, college major, geographical region, and race and ethnicity as some of the factors that account for the gender pay gap. (bit.ly/AAUW-pay-gap) The referenced page has FAQs on the subject; for more details look for the link to “The Simple Truth to the Gender Pay Gap.”