The clarity makes sense. I don't know exactly how the scaling works internally, but I suspect the higher you go above 100%, the less clear the element actually is. So a "Full HD" element at 100% will be 10% larger and 40% more clear than a Standard element at 140%.

Things like nameplates and the chat bubbles for NPC chatter are only scalable like this, so if you want to scale DOWN nameplates, you probably should go to a lower "High Resolution UI Setting."

It would have been nice if it just let you scale things up to 280% and dynamically switched whatever version of the element was used to maintain clarity. Instead, though, it still has a max of 140% but effectively multiplies this with the High Resolution UI Settings. So you'd get that 280% size max by having it set to 4K and then also scaling it to 140%.