There is an important logical difference between false and unknown: the opposite of false (“not false”) is true, while the opposite of unknown is still unknown.
For example, “1 = 2” evaluates to false and its opposite, “1 != 2”, evaluates to true. But “not unknown” is still unknown. If null values are included in a comparison, you cannot negate the expression to get the opposite set of rows or the opposite truth value.