Because {3,5} is a different dice roll than {5,3} - in the first instance, die 1 rolled 3 and die 2 rolled 5, whereas in the second instance die 1 rolled 5 and die 2 rolled 3. So the combination of 3 and 5 is counted twice because there are two different ways of achieving it.

{4,4} means die 1 rolled 4 and die 2 rolled 4. There isn't another way of getting both dice to show 4, so it's only counted once.