@Ghost
Late to the party re: fluency.
I think you misread the statistic, or the statistic was improperly reported. Susie Dent (she of 'Countdown' and '8 out of 10 Cats Does Countdown' fame) estimated that the average English speaker's active vocabulary, i.e. the words you use, is about 20,000 words, while your passive vocabulary, i.e. your active vocabulary plus the words that don't have you reaching for a dictionary, is about 40,000.
So yes, you absolutely do know 20,000 English words.Harry Potter and the Order of the Phoenix had 7,908 unique words. Extract a few dozen fake words (squib, muggle, etc), and you see just how many words you read without thinking about it. And there are so many words in English. We have words for everything. Plus, once you hit a certain point in vocabulary, you start being able to put together words by looking at their etymology and related words. For instance, say you know "hemo-" means "relating to blood." Suddenly you know hemoparasite, hemoparasitology, hemoparasitologist, hemopathology...
You've probably never seen the word "hyperpermeabilized" before in your life, but I'm positive that you instinctively know it means "the state of something that has been made really permeable." Because you know "hyper-" means "really," you know "-ized" means "done something to," and you know what permeability is.