The interaction between Darius and Last Whisper is misleading

This has been bugging me for quite some time now.

Darius's passive armor penetration from E (Apprehend) maxes out a 25%, which means you penetrate 25% of the enemy's TOTAL armor with physical damage. This is fine.

Battering Blows (the mastery in the Ferocity tree) gives up to 7% armor penetration at 5 points, meaning you penetrate 7% more of the enemy's TOTAL armor with physical damage. This is also fine.

Last Whisper gives 45% BONUS armor penetration, meaning you penetrate 45% of the enemy's BONUS armor, but none of the enemy's BASE armor.

I bought a last whisper on Darius and noticed that with Battering Blows and a level 5 Apprehend, my % armor penetration was 75%, which would only make sense if Last Whisper, Battering Blows, and Apprehend all gave the same type of armor penetration.

This would lead me to believe that either Battering Blows and Apprehend give Bonus Armor Penetration, or that Last Whisper gives Total Armor Penetration.

Or, and this is the most likely scenario, all 3 sources of armor penetration are working properly. That would mean the armor penetration indicator is misleading.

Does anyone know how Last Whisper interacts with Darius's E and Battering Blows? Cause I have no idea, and I'm bothered by not knowing.