Let’s take the first check.
null > 0; // false
According to the Spec, the relational operators
< send the statement through an algorithm called the Abstract Relational Comparison Algorithm to find out if the statement is
1. Call ToPrimitive(x, hint Number).
2. Call ToPrimitive(y, hint Number).
3. If Type(Result(1)) is String and Type(Result(2)) is String, go to step 16. (Note that this step differs from step 7 in the algorithm for the addition operator + in using 'and' instead of 'or'.)
4. Call ToNumber(Result(1)).
5. Call ToNumber(Result(2)).
6. If Result(4) is NaN, return undefined.
... read the whole story at blog.campvanilla.com.