I noticed that when I got Last Whisper on Darius after maxing Apprehend, his ArP was 60%, not 65% - does percentage ArP max at 60% or something? I suppose it'd make sense to prevent people from getting 100% ArP with Darius and 2 LWs.

Whats more important, why don't you have the 10% armor pen mastery? :O

I heard that Armor Penetration stacks multiplicatively, not additively, so there are diminishing returns. I'm not sure of the formula, so you'd have to check the Wiki for that.

EDIT: Two Last Whispers wouldn't work on anyone, they are Unique Passive, meaning only one Last Whisper armor penetration bonus can be had.

100% * 0.6 (from Las Whisper) = 60%
60% * 0.75 (from Apprehend) = 45%.
So the two together give 55% ArPen (effectively, Apprehend loses 40% of its Armor Pen because it's multiplicative).
With the mastery, 45% * 0.9 = 40.5% = 59.5 ~ 60% armor pen, the max possible to get in the game as far as I know.