Help with understanding of same type efectiveness

Hi there.

From some time now I started to wonder how Pokemon with certain type(s) is weak to their own type, it is not logical for me. And I don't have in mind weakness dragon to dragon and ghost to ghost. For example, why poison/ground Nidoking is weak to ground type, why rock/bug Shuckle is weak to rock or ground/steel Excadrill weak to ground. Sharing the same type should not have efectiveness x0?

Any explanations?