It's flat armor pen that scales with target's level.
"Flat" armor pen as opposed to "percent" armor pen.
Flat reduction means you subtract a flat number from the total, whereas percentage reduction scales with the total (so a larger total means more reduction).
15 Lethality reduces the armor of a level 3 champion by 10.
If you take away 10 armor from a squishy level 3 marksman with 37 armor, they are left with 27 armor, causing them to take 7.9% increased damage.
If you take away 10 armor from a level 3 juggernaut with 45 armor, they are left with 35 armor, causing them to take 7.4% increased damage.
30 Lethality reduces the armor of a level 18 champion by 30.
If you take away 30 armor from a squishy level 18 marksman with 90 armor, they are left with 60 armor, causing them to take 18.8% increased damage.
If you take 30 armor from a level 18 juggernaut with 100 base armor and 100 bonus armor, they are left with 170 armor, causing them to take 11% increased damage.
45% bonus armor reduction reduces bonus armor by 45%.
If you take away 45% of bonus armor from a juggernaut with 100 base armor and 100 bonus armor, they are left with 145 armor, causing them to take 22.4% increased damage.