If you want to sense-check the figures, IIE has data on international students in the US by nationality and level of study. If the main factor is really students with advanced degrees from US universities, dividing the number of graduate students by H1B applicants of each nationality should give a roughly similar ranking to the country-level success rates.
Adding a link would probably get my comment filtered, but the page title is: IIE Open Doors / Academic Level and Places of Origin
If you want to sense-check the figures, IIE has data on international students in the US by nationality and level of study. If the main factor is really students with advanced degrees from US universities, dividing the number of graduate students by H1B applicants of each nationality should give a roughly similar ranking to the country-level success rates.
Adding a link would probably get my comment filtered, but the page title is: IIE Open Doors / Academic Level and Places of Origin