Armor preventing Stone Spikes damage

Not sure if this applies across the board, but it happened to me a moment ago while playing Melvelous the Magnificent module.

Code:

Stone Spikes damaged Durazazul.
Unholy Curse added 3 damage.
Reliable Mail reduced damage to Durazazul by 2.
Resistant Hide reduced damage to Durazazul by 2.
Durazazul took 5 Piercing damage.

Stone Spikes is supposed to be piercing. Maybe that has to do with Unholy Curse being applied?

 

Are you mistaking piercing and penetrating?

 

Awww, yeah