This is the point. If you think about it for a moment, you’ll notice that you came across this phenomenon yourself while learning your second language (or third, or fourth, …).
I was watching a movie, Yesterday. In English if course which is my second language. After a while I noticed, that there were several words that I’ve never really heard before, even though I have watch that movie a zillion times.
Why is that?
Well, I suppose it’s just because that noise, so to say, never made sense to me. I just wasn’t able to hear the words. The conversation and its overall message was clear to me. You can get it even if you don’t get 100% of the words.
However, as I consider myself to be fluent in English, there is always “advanced information” no matter which level you are in, that I don’t know yet. The more time you put into learning your language, the more advanved you’ll become.
At some point you may ask yourself “What’s left to be learned?” And suddenly you’ll have this epiphany moment that there’s always more to come.