I've kept in touch with the Math/CS department at my alma mater and they've told me the male/female balance bounces around significantly. There have been years

I've kept in touch with the Math/CS department at my alma mater and they've told me the male/female balance bounces around significantly. There have been years where the department is 90+% male and years where the balance has been close to 50-50. I was primarily math and I took upper level math classes that were 75+% female but were closer to two-thirds male the following year. The year I graduated about 40% of the CS graduates were female, today it tends to be closer to 25%. I'll also add I went to a small liberal arts college with 60% women total.