Say you leave the gender ratio unspecified and instead decide based upon the interviewer's gut feeling. Then you'll bias the hiring process toward schmoozers with good social skills.
Or you decide based on which college the applicant went to. Then you bias it towards people who were willing to shell out for a prestigious piece of paper.
Or you decide based on whoever responds to your offer first. Then you bias it against people who have lives and better things to do with their time than refreshing their e-mail waiting for a callback.
Really, the only solution is to acknowledge that life's not fair, and people sometimes get things for completely arbitrary reasons. Which is really hard for a lot of people to do - it was hard for me - but you end up being a lot more successful when you don't think too hard about all the folks who get undeserved job offers and promotions and think more about how you can tilt the odds toward being one of the lucky ones instead.