I think I may also have somewhat of a blind spot here because I grew up with a mom who is a software engineer herself and I was bought a bunch of electronics/building toys by engineer relatives on both sides. When I was 13 or 14 I was given the parts for a computer under the instruction to put it together and make sure to dual boot linux. I knew a fair number of other girls my age whose parents really wanted them to be engineers/devs and did similar things, but a lot of them were uninterested and went on to happy careers in other fields.

The math vs CS dept thing is concerning because at the foundations they're very similar fields. It's such a strange phenomenon that my graph theory elective in the math dept was 30 or 40% female, yet algorithms was 5% female. Definitely at my institution there were structural issues in the CS dept that didn't exist in the math dept.