If u have a bonus atk for some kind of mob lets say FAIRY. Dont matter if mob are fairy or not, he will have the weakness with value 0 add, so u will add the atk bonus VS 0 defence fairy.
Correct way: Fairy vs Fairy
Wrong: Fairy vs Undead
The fix dont add wrong weakness with 0 value to mobs, fixing mob receiving dmg from other race types weakness.