I have my doubts about those numbesr. I know around 10,000 words in Japanese and there's no way I am even approaching below average high school students (ie the ones I used to teach).
Now that you mention that I remember 10,000 words is what JLPT 1 is supposed to test. Maybe it's slightly higher, but in any case, it's still a sticky mess to wade through. At any rate, the numbers I threw out there weren't supposed to be anything authoritative, just sort of a guide based on things I had heard before. I'd be open to more authoritative numbers, but I think my 50%/30% assessment isn't too far on the "knowing" side, and I'd be really surprised if any native English speaker could go through an unabridged dictionary (150,000 to 200,000 words) and average only 1 unkown word per page. I came across way more than that on this one alone. And that's from the Merriam-Webster Online Abridged Dictionary. Really, I'm not stupid.
I would wager as well that the number for college graduates is much higher as well. Know 20,000 words? I am sure there are far more than that in the reservoir of words that I actually use.
I want to come back and thank you guys for taking part in this thread! I've gotta keep coming back to it for learning reference... chock-full of good info! Domo arigatou gozaimashita!