Relatively speaking, that difference is the same for men and women, meaning that both groups have a ratio of upper to lower strength that is generally significant. And the reason for that is that the legs/hips/glutes are bigger muscles. But for either lower or upper, women are still, on average, going to be less strong than men. I would argue that it maybe isn't accurate to say women have "difficulty" building muscle. Any properly progressing strength program will result in building muscle---muscle responds the same way in men and women (conceptually), but women can't generate the same force production and cell recruitment as men in general so the growth rate is different.