I'm surprised. In a technical PM role a few years ago I found out that I earned 20-40% less than other PM's at the company. There were more than a dozen PM's and I was the only woman who was mid-level seniority. My male direct report made almost the same as me, while my female direct reports made a lot less. In my case it was not due to lack of initial negotiation, but that at each promotion I didn't get a sufficient…
I don't think it is strange at all -- after all, it is in the company best interest to pay the employees as little as necessary. Engineers benefit company by building products. HR benefit company by (among other things) figuring out how to save money on salaries.