I think American woman are more independent compare to other woman from those countries.
From the limited research I've seen on the subject sexism & independence of women is more closely tied to: availablility of birth control, unemployment, education and inequality of incomes. Which frankly the USA scores very poorly on. The USA was also governed in the past by the UK and has inherited a far share of the culture. Sure the American identity is more based on personal independence (regardless of the reality) than other former UK colonies which may explain the self-reported differences (if they exist - has there been a similar survey in the USA?).