I'm pretty sure that people use Calm Mind on Special Sweepers with High Sp. Def
And that people use Bulk Up on Physical sweepers with High Def.
They invest more EVs in the stat that is not boosted. For example, for Musharna, you would invest more in Def then Sp. Def cuz you're going to boost it by Calm Mind. However, Musharna's base Sp. Def is higher than its base Def.