(Had posted the below response on another thread on the same topic.)
I guess it ties in with all studies on the gender wage gap?
There are many indications of gender, by itself, not being a factor (see, for example, the study conducted by Uber on the earnings gaps between female drivers and male drivers in the USA. An eminent panel was invited to drive the study and their findings are pretty indicative that gender is not a direct factor for this gap).
See also the Simpson Paradox and its application to the question of the gender pay gap, in general - the BBC podcast More Or Less had a couple of very interesting episodes providing coverage on this topic.
I have seen the findings of the PM salary report - but I must admit that I have not dug deeper to see what the probable causes are. Saving Changes...
Of course this is of concern but I don't believe the PM profession reflects a wider gender wage gap than other professional roles so this is indicative of a broader, systemic disparity within organizations, industries or geographic locations.
What the PM community could do is demonstrate leadership in fairness by eliminating such gaps across all demographics (not just gender).