Insert "Thats not how that works, thats not how any of this works" meme here.
You are so very far off, its like you didn't even read what the item does. Did you just ask someone what it did and they answered "increased damage depending on health difference"? You even put the ability in your description and failed to read what it says. Yikes.
Okay, im slightly sorry for that (but not enough to not post it) but here it goes.
Grants UP TO +15% physical damage. UP TO. As in, this is the MAXIMUM. SO if an adc with 100 ad buys this item, and the opponent has no armor, and has max health (not current) greater than or equal to 500 more than you, they will hit you for 115 damage. This is true if the tank has 500 health more than the buyer of this item (usually the adc) or 300000000000000 health more than them. The bonus damage is only less when the tank has less than 500 max health than the adc.
Lets also talk about another thing. The armor pen does NOT increase at all. At any time. It only serves to make the damage that goes through to the tank more effective. It doesn't increased based off Max Health. It stays at a constant 45% from the time you buy it till the time you sell it (don't sell it).
This item isn't nearly as strong as you think it is, and with proper tank itemization, you'll still be able to handle yourself vs the adc for a while. Do note, however, that a full build adc (and some not full build adcs) will kill you in a 1v1; a tanks job is to lock down the squishy targets (the adc) for the rest of his team to kill. So full build vs full build you lose, regardless of what you've built. Im generalizing there but thats generally the story.
