Could you please explain some of the game mechanics?

1. It accumulates.
2. It is subtracted.
Don't confuse resistance and magic defense. Resistance gives a chance to completely deflect magic, as if the defense were 100%, while defense only reduces the damage by n percent. Therefore, even when resistance triggers with a developed Warlock skill, damage is present, but still less than the base damage.
3. Read the manual; these are basic principles. Good luck.