Why does Obama pay his female staff less money then his male staff. I thought he was for equal pay ?

When all was said and done, it was calculated that Obama paid his female staff $0.83 as opposed to every dollar paid to his male staff. McCain on the other hand paid his female staff $1.04 for every dollar paid to his male staff. How come? Because McCain has 13 females on the top 20 of his highest paid staff.