Armor/Magic Pen Bug?

Not sure if this is intended or if it is a bug but the armor penetration from the mastery Battering Blows is stacking additively with the Last Whisper items for a total of 52% armor pen.

Piercing Thoughts is stacking multiplicatively with Void Staff for a total of 40% magic pen.

I know it's bonus % armor pen from the Last Whisper items, this may be why there is a difference, but I was curious as to the reasoning behind this difference.