Doesn't seem like much of a paradox. By definition, people who immigrate are:

--smarter --wealthier --better educated --more ambitious. take-charge people

than the general population. These are people who have taken the initiate to uproot themselves from their current NATION, leave everything behind, and negotiate a difficult and expensive immigration process. These are people that are deemed worthy in the receiving nation: people who have advanced degrees or other skills, for instance (legally immigrating to another country is basically impossible if you are a ditch-digger).

Why wouldn't these kids do well, as a rule? Their parents are people who valued education and who would enforce that in their kids.

And then in the second and succeeding generations, the effect disappears.