One of the reasons students may do better is language. Being raised and attending school in Canada would be an advantage for the York students and a disadvantage for the parents. Also the age factor plays another role, It's easier to learn a language when you're younger. This all relates back to nature vs. nurture because it proves that your environment plays a big role in your intellectual ability. An Example Moghaddam gave in the book is mass intelligence testing of military recruits during World War I (1914–18).
During World War I millions of recruits had to be quickly assessed, trained, and placed into position in the military. Many recruits were illiterate, and many were immigrants with little knowledge of English. pg.98
the examination results showed significant differences between ethnic groups of immigrants, with Northern Europeans scoring higher and the Chinese lower than others. the outcome of this changed the immigration policies, making it much more difficult for people to migrate to the US.