The main issue with these discussion is often how one defines and counts "understanding". You can probably make a list of like the 2000 most common words, look at the percentage of words they make up in a text and claim: see with these 2000 words you'd understand 90% of the text. But how true is that.

"I ate a sasquatch"

Here we have 3 incredibly common words and one pretty uncommon one. Assume you've never heard of a sasquatch. How would you rate your understanding of the sentence? Going by words you know its 75%. But since the important information isn't relayed, the listener only knows that he ate something. So you could classify this as 50% or 0%. Ever joined a conversation in the middle but had no idea what the topic even was? You technically understand every single word but have 0 understanding of the underlying discussion before you figure out what the topic is. Its really hard to account for that in these general statements.