If you actually do the math on it, when you start with 40%, you gain 3.3% per level (3.3 x 18 = 59.4. A little off, but rounding it upwards will give you the 60% to add to 40 to get 100%)
When you start with 60%, you get 2.2% of it back per level (2.2% x 18 = 39.6. Again, very tiny amount to be off by, but the math is sound)
Let's compare at levels 6, 11, and 16.
40%: 40% + (3.3 x 6) = 59.8% effectiveness. If you had 20 Lethality, you would be getting 11.96 Armor pen out of it.
60%: 60% + (2.2 x 6) = 73.2% effectiveness. With 20 Lethality, that's 14.64 Armor Pen. A whopping difference of 2.68 Armor Pen.
40% + (3.3 x 11) = 76.3%, or 15.26 out of your 20 Lethality
60% + (2.2 x 11) = 84.2%, or 16.84 out of 20 Lethality.
40%: 40% + (3.3 x 16) = 92.8%, or 18.56 out of 20 Lethality
60%: 60% + (2.2 x 16) = 95.2%, or 19.04 out of 20 Lethality
And of course we know that at level 18 the values balance out to the same thing. But look, really LOOK at these numbers. This was not a huge buff. This was a slight buff to a stat that was actually pretty damn fine by itself. Nobody realized it was alright, complained about it, and now everyone is just noticing what everyone who was using Lethality before the buff already noticed.