The thing is, 90% coverage is not that great. What happens is that you understand common words that make up for a lot of structure, but when an uncommon word appears it's probably important to the sentence. For example, "son, if you go to the plumbf tomorrow morning don't forget to pick up some zlonks." 98% is closer to what you need in order to read a text and have an idea of what's going on. See this article: https…
Edit: Doh, the OP link actually includes this statistic.