Holy wowsers. I grew up in England, and i only scored 14,800 on the test! Although English isnt my native language, i did speak it from a very early age.
I was completely honest scoring 16,000, English is my one and only language. Depending on your field/background you could understand more words than this test suggests. Looking up the words that i did not understand coupling them with there relevance and the shear volume of test words or lack thereof makes this test inaccurate in my mind.