America has not just a lot more people, but a lot more immigrants, refugees, and more.

In a more homogenous society, such as Poland’s in comparison to the United States, it also in my observation trends to be easier for wealth distribution to flatten.

Maybe the US has more kids per capita too which would also affect that value depending on how it is being measured. Not to say that anything is being hidden there, just something to think about that may make you interpret the data differently or think about it differently.