The scale of mobility is different, in the USA you can be much more wealthy. That's thrkwd direct comparisons like this off. If every is relatively poor then mobility is really high and uncorrelated with parent wealth.
While those rags-to-riches cases are true data points, they are not representative of the population sample. They are the exception despite the strong correlation, likely due to another contributing factor (sheer random luck, great financial earning abilities, etc.).
