[Realism]

And plus, the reason the kid knows more Japanese than the father is a simple one.

Because she was exposed to large amounts of Japanese all day long, maybe by watching TV or hanging out with other Japanese kids.

So she has exposed herself to Japanese for 3-4 years, which is like 20,000 hours or something.

She she has been exposed to Japanese for 20,000 hours already unconsciously and consciously.

While her dad has lived there for 10 years.....probably rarely listens to Japanese on any given day, and just reads, talks, and thinks in his own language all day long.