So, the very first result on Google gave me this:

https://www.statista.com/statistics/1126823/worldwide-develo...

Which doesn't cite its source directly, but it's probably the Stack Overflow developer survey:

https://survey.stackoverflow.co/2022#section-demographics-ge...

Admittedly, that's not a great source for objective information since it is going to be skewed by the demographic that is responding.

However, this is the first time I'm hearing that the number is as high as 20%. In my career, the most women on the same team as me was literally 1, as far as I can remember.

Why does my experience differ so much?

This is true for employees in mainstream careers. Unfortunately it seems to not hold when looking at major open source projects, where gender diversity is routinely below 10% female. The picture gets even worse when proportioning representation by the quantity of changes/commits, where female representation is well below that.

https://arxiv.org/pdf/2103.08763.pdf