It’s that… Thing… Where if you know stuff, you realise how much stuff you don’t know. And if you don’t know, you don’t know you don’t know.

So because they know English a bit, but it’s not as much as their primary language, they realise how much they don’t know.

Whereas people who only know English don’t know how bad they is at it.

FYI I only know English, but I’m pretty great at it.