If they're actually working in the same jobs, wouldn't this discrepancy show up in the statistics? If all the male senior developers are driving a hard bargain and getting $220k, but the female senior developers aren't and are only getting $200k, it'll still show up as a 10% difference in an apples to apples comparison. The fact that such apples to apples comparison shows minimal difference either means such effect is tiny, or there's a bunch of effects working for females that's canceling out the "males bargain better" effect.
However it is also worth considering that apples to apples is hard to compare and may have a latent bias you accidentally brought up by adding the adjective "senior". What if, and this is what I've seen, females tend to promoted and said titles slower/later. So a 32 year old female may be stuck at non-senior $180k while the 32 year old male cohort she started with got the senior title 1/2/3 years before and is at 220k.
Some of this is still of the same flavor of "not demanding it" as aggressively as a male might.
Otherwise, yes most of the time people point at far bigger pay differentials in stats that are driven by apple/orange compares like comparing male majority SWE jobs and female majority teaching/nursing jobs, which duh, high %% difference.
My wife will spend many evenings arguing with me about why she can’t just ask her boss for a raise (rather than venting to me about it for months) before finally agreeing to do so.. and winning.
However, In my experience my female colleagues doing similar things are generally under-compensated BUT, and this is a big BUT.. they also are less aggressive in asking for money. I have had this discussion with my wife, and every few years she does work up the energy to have the "give me more money" discussion with her boss, which is almost always followed by a best-in-years % raise at next compensation cycle.
I coach some of my former junior colleagues too and the women are generally paid a little bit less than their peer male coworkers, but also their reaction to this injustice is a lot less "I'm marching into my bosses office Monday morning and demanding a raise" than a male in the same seat would have.
And then yeah there's also some lingering biases.
If I had to make up some % allocation for pay differentials that I've seen, it's probably - 50% job choice / 40% career management / 10% bias. But who knows.