Because in the first 3 generations, that's how it works. Obviously, since the 4th generation reworked the formula to be based off the Physical/Special attribute of the attack, the part of the guide that refers to damage done based on the type of the attack doesn't apply. That doesn't mean it's wrong, it simply means you're misusing the information.
latina mortua lingua est // ud mortua ud ea possit
prima meas ancestras necavit // et nunc me necat