She mentions her field of STEM (mechanical engineering) has a particularly low percentage of women among its members, and cites another site. On that site, there is this interesting list: 39% of chemists and material scientists are women; 27.9% of environmental scientists and geoscientists are women; 15.6% of chemical engineers are women; 12.1% of civil engineers are women; 8.3% of electrical and electronics engineer…
Women make up about 50% of undergrad chemistry degrees. On the other hand, they make less than 20% of physics/math degrees (engineering funnels). I don't have any real idea why women are so low in math and physics, but I suspect you'd have to look back much further in time.
I am very inclined to think this is mainly a matter of preference. On the other hand, maybe the situation in the US is completely different, and Slovenia is just so much more equal when it comes to sex (e.g. the unadjusted gender wage gap is only 2.5% for Slovenia).