Why are you assuming the ratio of female to male sf and fantasy writers is the same in 2013 as it was in 1999?
For that matter, an analysis of the membership of SFWA is not nearly as good an indicator of the number of books published by gender as actually looking at the number of books published. You don't need to re-qualify for SFWA membership once you've qualified, so people who haven't been published for twenty years are just as likely to be members as people who published this year. Nor does every science fiction or fantasy writer join the organization. The membership is not necessarily an accurate reflection of the current makeup of the field.
no subject
For that matter, an analysis of the membership of SFWA is not nearly as good an indicator of the number of books published by gender as actually looking at the number of books published. You don't need to re-qualify for SFWA membership once you've qualified, so people who haven't been published for twenty years are just as likely to be members as people who published this year. Nor does every science fiction or fantasy writer join the organization. The membership is not necessarily an accurate reflection of the current makeup of the field.