It doesn't, it has slightly V shaped sound with a bit emphasized treble, not much but it's never too quiet and it's not so smooth at all, let alone very smooth, at least not mine that are genuine 100%. Highs are not piercing and unbearable but they are far from laid back smooth quiet, light year away from that.
Maybe there were different versions of IE80? (I know there is some Asia version and US version...) since what you said is pretty opposite from my experience with earphones and my IE80 that I use every day for 2 years now
EDIT: Just read several reviews... everyone says highs are bit emphasized and high, few even said they are too harsh tho I don't think so... not sibilant but for sure a bit emphasized, especially comparing to mids. Nowhere near smooth or quiet, at all.