Some funny things I noticed about Armor Penetration (Part 2)

The first part of this thread can be found in the Tips and Tricks Section, where I talk about some major differences between Flat Penetration and Armor Penetration. I really hope you check it out, as it provides some context for this second point about Armor Penetration.

So, onto the second point: Assassins and Penetration. I just explained to you guys how Flat Penetration is the weapon you use against targets with low armor values, and what happens to be the general class in the game who's entire job is to shoot down low armor targets?

I just find it close to outright hilarious that Percentage Penetration items, items designed and mathematically reinforced to be best against targets with high resistances, tend to be so damage per gold efficient on most assassins in the game against low resistance targets. I mean, why is this? Numbers prove that there should be distinct niches for both kinds of penetration in this game; where Flat Penetration is the stat that you invest into to punish people who don't build tanky or for Assassins to optimize for seeking out their squishy targets but falls off towards high armor targets, and where Percentage Penetration is the stat that you invest into to keep the damage dealers capable of fighting tanks and giving people the option to be dedicated tank busters should they itemize for it while sacrificing their viability to fight low armor targets. Why is it then that the Last Whisper or the Void Staff seem to be catch all items to fight any kind of enemy regardless of how much armor they have while still being extremely cost effective against even the squishiest of targets?