you prettymuch have proven Critmaster Garen right here.
The graph shows that with 1000 health:
an AD hits you first and_(going by your numbers he hit's for 1000)_, to use 100 Armor increments, leaves you with the following amount of health based on your Armor:
100 Armor -> 500 health remaining
200 Armor -> 666 health remaining
300 Armor -> 750 health remaining
400 Armor -> 800 health remaining
So vs a 0 armor target, the adc needs 1 hit to kill in this example, vs a 100 armor target he needs 2 hits, vs a 400 armor target he needs 5 hits.
So every 100 armor increase your effecktive health by 1000 in this example, and thus increase the number of hits that the adc needs by 1, which looks like a perfect ecample of linear scaleing without any diminishing returns...
in fact, the armor the target has, the more effeckt LW has.
We again use 1000 health and 1000 damage as example.
If you have LW vs a 250armor target, you reduce it's armor t by 100, so 150 left( i use 40% aroen for simplicity, with the masteries+LW you'd be at ~39%)
This means you need 2.5 rather than 3.5 hits, so the pen of LW would equal 1000raw damage for this scenario.
If you have LW cs a 500 armor target, the armor gets reduced by 200, down to 300.
So you need 4 rather than 6 hits, which means the pen of LW equals 2000raw damage for this scenario.
note that relative to the scenario without LW, you need 33,3% less raw damage with LW vs 500 armor, while it's only ~28,6% vs 250 armor.