Statistics
I can't find hard numbers for this, but I bet if you factor in American minorities, illegal and legal immigrants and other such people into these equations, that a lot of this discrepancy is explained. It's kind of like what happens to the gun statistics about how many "children" die from gun violence when you go in and factor out the teenage gang-bangers.
The U.S., a huge, churning melting pot, is constantly compared in these statistical wars with nations that are largely culturally and economically homogenous. Japan is a typical selection. In Sweden, "minorities" are people like Finns, Roma, and Jews.