I find it more likely that 16 million Americans decided to take the piss when answering that particular question, perhaps offended that such an obviously dumb question would be asked of them with a straight face. I would be.
16 million Americans have, by definition of the scoring mechanism, IQ 70 or lower. I suspect the only reason the number thinking it’s from brown cows isn’t higher is all the people who don’t think milk comes from anywhere but the supermarket selling it.