There's a ton of possible interpretations here - could simply be that more women just generally have other things to do with free time than men. Or maybe it's that women are genuinely scared off of open source development due to problems. Or an issue with the studies that get those results as a lot are based off username, which might be a bad metric. Lots of possibilities.
No good way to know by speculating.
I think that can mostly be diversity hiring. A company can hire women/minorities to increase diversity. A FOSS project can't accept code on the premise of diversity. I think the narrow scope of most FOSS projects (or the individual components that contributors wind up specializing in) also makes most of the benefits of diversity less relevant. Diverse teams do better on diverse tasks.
(I don't think diversity hiring is a bad practice as long as the people hired are qualified for the job)