Does Lethality stack effectively?

Hello!

I wanted to ask you, if Lethality can be stacked effectively. I tested it out in a custom game, and the total amount of Lethality indded stacks up if I get e.g. a "Duskblade" together with "Youmuu's Ghostblade". However, has this any benefits? Hovering over one of those two items with will show me additional info information and it says me that I "Negate X Armor". X didn't increase when I purchased another Lethality item.

So the Lethality goes up when I purchase multiple different Lethality items, but doesn't actually benefit me with additional armor penetration, doesn't it?