I had a friend in college whose mother was an immigrant and whose father had grown up bilingual and spent years in the mother's county. My friend was able to hear and understand the mother's tongue with apparent fluency, but had an extremely difficult time forming real sentences in the language, even though he'd heard and understood it for his entire life.

I think there's a significant difference between consuming language and producing it, whether that's written or spoken. It might explain the number of people I see these days who don't understand basic English grammar and make words plural by slapping on an apostrophe and an s.