Good question, but a small (yet significan) mistake in the explanation.
NULL does not represent the value 'UNKNOWN'. NULL is a marker for a missing value, without any indication as to why the value is missing - the value being unknown is one of the possible reasons, but far from the only one; not applicable being the second-most common reason, and a whole bunch of less common reasons to follow.
Since a comparison with a missing value can never result in either of the truth values True of False, such comparisons will result in the third truth value, Unknown. So while NULL is not the same as Unknown, it does have the effect to make 1 <> NULL result in Unknown.
I have explained this in far more detail in a four-part series on my blog:
* NULL - The database's black hole
* The logic of three-valued logic
* Dr. Unknown, or how I learned to stop worrying and love the NULL
* What if null if null is null null null is null?
I must say that I am also very surprised (and disappointed) by the amount of incorrect answers. Almost 25% of respondents expect NULL to be returned as well - much more that I expected, because this is far from the first time that the effects of NULL in comparisons have been tested in the QotD.