Description; Increases M.Attack by 50%, P.Def by 50% and M. Critical 50%.
Why does not change the effect of P. Def and M. Critical in the status when you activate the skill, put the skill without buff does not raise the amount of 50%, gives less than the description.
The skill you are talking contributes to skillPDefendPer, while here are a few things that contribute to skillPDefendDiff:
Superior Robe Mastery (1.9k @ level 104)
Abundance (~1k)
Diamond (~1k @ level 3)
R99 set +6 bonus, longing (+60 each)
Augmentations (value depends on augmentation; talking about non-skill augmentations ofc)
(cba to find and list more, just from the top of my head)
So it is realistic to have anywhere between 2k and 5k P. Def. that is completely unaffected by this skill.
As for M. Crit., there's also some flat adds you may have:
Desire/Longing (+10)
Sub - Critical Rate Increase skill (+10 to +40, depending on level)
Augmentations (non-skill)
Buster type weapon >= +4