I'm not sure what these numbers even mean, in the long run. I got my B.S. in civil engineering in 1993, and my M.S. in 1999. There was a definite increase in the percentage of female students throughout my undergraduate and graduate years in school.
And yet when I took my state board licensing exams (Part I in 1996, Part II in 1998), no more than 5% of the people taking the exams with me were women.
I think that means that the women pursued engineering careers that did not need State Licensing, or changed careers. Few engineers get their state licenses. It's only if they are going to be consultants where they have to sign off on their work, have careers as expert witnesses, or city or state department heads that they bother.
My husband finally let his license lapse because nobody ever was very impressed by it and we own our own company where it is not needed and he got tired of paying the dues. Neither son has even bothered to go for his. That really irritates me because I was always impressed by the license and my husband's ability to sign PE after his name and afix a nice official stamp underneath! And I remember how hard he studied for the two tests. His was a good license too with reciprocity in every state of the union.