This low percentage of women editors is a rather puzzling pattern.
I mean, is film heavy? (That's actually not a totally stupid question: a full reel of traditional 35 mm film is quite heavy, but I have no idea whether apprentice editors are expected to do any heavy lifting of complete reels, the way apprentice cinematographers have to climb ladders carrying heavy lights.)
Film editing is a highly respected if not well understood craft. Oscar voters assume that good movies are good in sizable degree because they are well edited:
Nominations for this award are closely correlated with the Academy Award for Best Picture. Since 1981, every film selected as Best Picture has also been nominated for the Film Editing Oscar, and about two thirds of the Best Picture winners have also won for Film Editing.
On the other hand, film editors are almost never singled out for achievement in an otherwise mediocre film, the way actors often are.
The two most honored editors currently working are Spielberg's editor Michael Kahn (eight nominations, three Oscars) and Scorsese's editor Thelma Schoonmaker (seven and three).
Schoonmaker's career is of interest. She edited a Scorsese student project in the 1960s and earned an Oscar nomination for editing the concert film Woodstock way back in 1970. But she couldn't get into the editor's union for a decade so she was blocked from working on Hollywood features throughout the 1970s. She finally got her union card (she thinks Al Pacino pulled some strings for her), and her first feature with Scorsese was 1980's Raging Bull, which would be high on anybody's list of superbly edited films.
She's edited only one movie since for anybody other than Scorsese, but has edited all of Scorsese's pictures. This may explain something about why Scorsese, who looked in the late 1970s to be headed toward the usual career of a director who burns brightly for just a few years, has made so many comebacks.
IMDB has some quotes from Schoonmaker on the gender question:
I think the women have a particular ability to work with strong directors. They can collaborate. Maybe there's less of an ego battle.
I'm not a person who believes in the great difference between women and men as editors. But I do think that quality is key. We're very good at organizing and discipline and patience, and patience is 50 per cent of editing. You have to keep banging away at something until you get it to work. I think women are maybe better at that.
People expect artists to be too normal, I think. I've been around enough of them now to see that they're very extraordinary human beings who behave differently than ordinary human beings. If they weren't as sensitive as they are they wouldn't be great artists. They are not the same as us. People should just learn to accept that.
Schoonmaker has long reminded me of Vera Nabokov, the classic example of the old, extremely unfashionable saying, "Behind every great man is a great woman." Vera put up with Vladimir's eccentricities, organized every aspect of his life, accompanied him to all of his lectures at Cornell, sitting in the first row to keep him on top of things, and even drove the nondriver on all of his butterfly-collecting expeditions across the West. Throughout decades of obscurity and economic deprivation, she remained convinced that her husband was a genius. Suddenly, in 1958 when he was 59-years-old, the whole world came to agree with her.
So, reading up on Schoonmaker on Wikipedia, I was struck by:
Schoonmaker was interested in a career in international diplomacy and began attending Cornell University in 1957, where she studied political science and the Russian language. (She attended classes taught by Vladimir Nabokov.)
In other words, Schoonmaker saw Vera Nabokov at every class.
P.S. Another director-editor team was Quentin Tarantino and Sally Menke, who died hiking in Griffith Park on a 113 degree day in 2010. Thus, as Uncouth Reflections noted, Django Unchained was the first Tarantino movie not edited by Menke. Surely, she would have talked Tarantino out of including in the final release The Worst Scene Ever: you know, the long episode toward the end where Quentin shows up in Mississippi in 1858 talking in an inexplicable Australian accent, and looking so physically decayed he epitomizes Orwell's line that "At 50, everyone has the face he deserves." (Or in QT's case, at 49.)