MP: Setsumi - Come for the game, stay for the drama!

[Kuritza]

Dice rolls are open-ended here, afaik. On a roll of 6, you roll 1d6 one more time. So actually, MR 18-19 gets penetrated routinely, even without penetration items.
...oh, and maybe some spells get some penetration bonus themselves, as an undocumented feature. At least SS, control and charm tend to work. Maybe its just Murphy's Laws though.