This is also something I have observed in my job. I am in a typical enterprise software shop and I feel like women are more likely to see programming as "just a job", while men are more likely to put more passion into it. But most people in general see it as "just a job", so maybe hobbyist female programmers are just rare because female programmers in general are rare.
I'm a woman, I program professionally and as a hobby, I have friends who do too. I think maybe the biggest problem is visibility. I work on things and have a lot of code on github but I don't really get involved with open source projects in my free time.
For me it's a combination of not wanting to deal with the very abrasive personalities idolized in some open source communities and just preferring to spend my free time hacking on throw away code to experiment with new ideas. I write clean readable tested production code at work, I want my hobby programming time to focus on impractical educational exploratory programming.
It also just feels like I have a lot less time to write code than my coworkers, even though I'm the only person on my team without kids. Traditional gender roles are evolving but it still feels like I'm left with more life stuff to deal with.
This is also something I have observed in my job. I am in a typical enterprise software shop and I feel like women are more likely to see programming as "just a job", while men are more likely to put more passion into it. But most people in general see it as "just a job", so maybe hobbyist female programmers are just rare because female programmers in general are rare.
Who knows.