Codes of the Tank

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If you have a % reduction before a flat reduction, on a curve that's sloped the way the resistance -> damage mitigation curve is, then the added value of penetrating lower levels of armor is exponentially more than the value of penetrating high levels of armor.

E.g. taking a tank from 350 -> 314 armor with the 36 lethality on two items does jack shit, and your damage against them will barely increase.

If you first have a teammate stack BC on them, they only have 266 armor, so your 36 lethality drops them to 230. Going from 266 -> 230 armor gives you much more damage against them than dropping them from 350 -> 314.

But, say you have LDR as well, now they only have 350 -> 266 armor due to BC -> 172 due to LDR -> 136 due to your lethality.

Dropping them from 172 armor to 136 is massive. Really, taking them from 350 -> 136 is just depressing, and it only takes 2 items the assassins were already going to buy, a staple item for two classes of characters that the top or jungler will virtually always have in every game, and then an LDR which more people should be buying anyway, and armor basically doesn't exist.

250 bonus armor is made into literally 35 that's 4,300 gold worth of pure armor tossed down the drain.

That doesn't mean that it scales with %pen, lethality isn't reducing more armor thanks to %pen, it's reducing the same "fixed" amount, you just added 2 %pen items that aren't supposed to be for assasins. Also getting 36 lethality would require you to buy 2 letality items, that's a total of 4 items you're using to reduce your enemies armor, the 5th is boots and then you've got only 1 slot to compensate every other weakness you have.

Lethality and %pen don't affect hp.

Going from 172 to 136 armor is not a massive difference, it's a small one, lethality is BAD against anyone with more than 100 armor, the massibe difference is going from 350 to 172 (50% of the original armor) but that happens because your enemy buyed 2 items to counter armor.