I remember reading a study about gender pay disparity (where it actually exists between similarly skilled people in the same industry) was because the female members of the group were less likely to ask for pay raises than their male peers. They may have felt more intimidated or less entitled to simply ask, possibly as a cultural side effect (maybe as a result of childhood stuff like the way males are highly competitive early on).

I know a female software engineer friend of mine had read about that and got inspired and ended up asking for a serious raise when she got a new contract. I remember being both surprised and jealous the person said yes and took it as a life lesson.

I also remember tptacek on here always talking about the value in asking for more money as a software person, product, or consultancy because you're probably worth it.

So as the other commented mentioned the qualities of new immigrants might certainly select for this sort of thing, ie, people with high-motivation, bold life plans, taking advantage of their opportunity to really buy into the American dream, etc. That combined with a local-cultural confidence thing could certainly help explain it.