Sure, you know a lot of words and don’t have grammar problems, but do you really have a deep knowledge of English and the ability to connect idioms with phrasal verbs, use the right slang word at the appropriate time?  For example, here’s a phrasal verb: tell off - can you match it with one vocab word? an idiom? a slang word?  Your understanding English at a high level when you can connect phrasal verbs, idioms, vocab and slang.