My wife's niece (local HK) has repeatedly showed me her diplomas depicting her straight A's for English. At the same time she keeps making grammar and word choice mistakes any Brit/Yank/Aussie would feel ashamed off doing while being drunk.
On the other hand I have a friend (local HK) who spend several years in the UK studying. His English is superb.
They are both middle class and absolutely didn't had tutoring 24-7 since their birth.
I think openness to experience and the need to use a language make a big difference.