You're right. However, in my experience, there have proportion of female PMs to male PMs than women engineers to male engineers. (Admittedly, My experience is a poor substitute for actual research :) ) And...PMs tend to have more influence on the user experience. If that is what we're trying to solve for then more women PMs are what we need. Again, not saying that we don't need more women engineers. I just don't think that engineers have as much influence on the user experience.

As far as PMs go, I think it has more to do with the type of business the company is in. I observed a higher number of women PMs in a Saas company I worked for that was focused on HR related software then the company I work for now, which tracks equipment used in construction, farming and energy.