What exactly is that, anyways?
I suppose I always connected it with being able to speak like a native. The thing is, in a second language, you are always aware of the things you CAN'T say or communicate, the vocabulary you HAVEN'T touched. Language is about trying to express oneself, which can be fairly difficult in my native tongue, so perhaps the fact that it isn't always eloquent in a second language doesn't immediately translate into beginner status. Claiming to be fluent seems pretty steep to me, since there is always more to learn! But, I can open a bank account, get telephone and internet service, argue with the post-office lady when she won't send my box for the flat book rate, read French literary classics, participate in dance and music classes, and converse with waiters at the restaurant (some of which I've never tried on my own in English). When do you reach the coveted title of FLUENT?
Immersing yourself in a foreign society results in hearing the foreign language around you every day. If you are attentive, you'll hear the same conversation you need to have repeated ahead of you in the line at the grocery store, post office, butcher shop, or at the next table over at the restaurant or café, and thus learn common phraseology and cultural nuances that you could never learn from a textbook. But there is SO much of that language around you--Immersed is a term usually used for water--you are simply swimming in the language, sometimes all you can do is try not to drown. You reach out all around you, but there is simply not enough time to examine each water molecule,(or every new vocabulary word).
I propose that while living, and succeeding, in a foreign culture and language, it is often 'grace à' what I call a borrowed vocabulary. There are loads of words one is able to use because the word has just been used on you. Whether or not you'll remember and be able to use that word a few days later often depends on if you hear it again. For every action there is a reaction. I think the same principle applies with languages. When I left Chinese society after a year of immersion with them, I found my vocabulary pool somewhat reduced. When I came to France after years of studying the language, I found my vocabulary pool suddenly greatly enhanced. Succeeding in a language demands interaction and interchange; its a game of exchange. Which is why, I'd wager, that one is always more successful at being 'Fluent' when immersed in the language than otherwise.
Perhaps there is 'borrowed vocabulary' in a native language too, that's another thought to ponder...