Great pressure to hire women in STEM, and there are fewer of them. I’m not sure about the specific case you are mentioning, but most savvy employers are more cautious because Title VII works both ways.
At one company I worked for, we would make base salaries similar, but we would definitely give women larger hiring bonuses. (We would also give larger bonuses based on college, etc.). Perhaps a women would get 3 months hiring bonus and a man 1 month.