The rows (1, NULL), (1, NULL), and (1, NULL) - each NULL signifies an absence of a value - these NULLs may actually be later filled into (1, 1), (1, 2), and (1, 3). That's why may not know if any two NULL values are equal to one another.

In some cases the above is actually important, whereas in others you may want to treat NULL as a "value" instead.