~60% of the developers on my team are women. It's not magic. Also, in a big company most employees are not engineers, so the relative lack of female CS grads shouldn't mean there are so few women that you need to hire them for events.
If you are hiring 100s, or even thousands of engineers you are bound to the supply of the market. Unless you do some straight out hiring discrimination, and refuse to hire males. Similar to some Vegas clubs (they wont let males unless they have a female with them). Good luck building a successful business with those values.
It is Econ 101 dude... or dudette (not sure you are guy or a girl)
P.s. I personally would love to have seen balanced genders (working for tech companies sometimes it feels like you are in the army), but that's probably a utopian ideal that is not going to be fixed in my lifetime.
It often comes back to this "pipeline" problem, but we need to think of our companies as being part of the pipeline. I don't have a CS degree -- I came out of college with a BA -- but I landed a firmware programming job, 2 jobs and 7 years later I am experience and "senior". If you're willing to train smart people, you can sideline the pipeline issue.
Which is leagues above the vast majority of engineering companies, as well as the overall population within the software engineering community at large. Yours is an outlier, and an obvious one at that.
So I'm boasting a bit, not claiming this is normal, but just to say... come on guys, think bigger.
The pipeline isn't sufficient to explain the gender ratio.
I just don't see any data that supports what you're saying. 20% is the _upper_ bound.