"I think on average the difference between a high school student from Russia and US is negligible." That's difficult to believe. I went to comparable level early education schools in Russia and the US and observed stark differences in the quality of math education. Math culture is a thing, and it varies between countries, affecting the education system on the whole as well.
I think OP's comment focuses on the average outcome and your comment is about the education process. I think both are right. While there are stark differences in the process, the average outcome is probably not much different. Students not in the US can do harder problems in their final exams but how much of that comes from actual understanding of math. How much is retained a day or a month after the exams? This is about the average student, mind you. From my short talks with math educators in the US and in a Soviet-like system, I got the impression that the average math student is a mess in math in both cases.
There are probably more people on the long tail of math talent here so it's understandable their impressions on the two systems are much different. OP's comment also talked about the pipeline for gifted kids and he concurred that the Soviet style is effective for training those students.
