I was comparing two characters, a Nightblade and a Templar in pretty much the same gear and exact same champion points set up and couldn't figure out why their penetration stats were different. Turns out the penetration bonus from Master Assassin, which (should) only work when flanking, shows up as standard penetration on the character sheet. Should that be happening?

Edit: Tested on a dummy, and the passive works as intended. But with the character sheet misleading me I was actually under the penetration threshold on my Nightblade, which is annoying since I like to solo dungeons and rarely end up flanking mobs there.

Edit 2: Turns out the info on the character sheet automatically changes depending on whether you stand in front or behind a target. Confusing.