Why do some Pokemon with the same type sometimes have them swapped?

For example, in the case of Calyrex(Psy/Grs) and Iron Leaves(Grs/Psy), I take it to mean that the primary type represents the type with which the user is more proficient while the secondary type represents body composition/defense or something

How would you feel if Gamefreak made it so that only one type was counted towards a pokemon's weaknesses while both still count towards STAB?(Kinda like perma Terastalization)