Actually, the studies adjust the incomes for purchasing power parity, so it is not just nominal income. Costs of living and similar are built into the relative income quintiles and deciles when comparing between countries.
The US also has a wider income range because of the enormous differences in cost of living within the country itself, in large part due to its size, which skews the result. The top quintile in one region of the US may be the middle quintile in another.
The US also has a wider income range because of the enormous differences in cost of living within the country itself, in large part due to its size, which skews the result. The top quintile in one region of the US may be the middle quintile in another.