Tell me, /vp/, how do you think GF decides on what weaknesses and strenghts a double typed Pokémon has?

Because I find it rather random. Bulbapedia has a rule described, but there are Pokémon that contradict it.

For example, Sableye and Spiritomb don't have weaknesses, but Archeops is fucking weak to himself (Rock). Why is the weakness valid in one and no the other?