Most high DPI web-browser/software run into the main problem that the world runs on <= 96dpi screens. So for the act of sainty they all throw their arms up and say, `We assume the device is 96dpi and we scale a pixel to that`.
Consider it this way. When you had a A4/A3 size paper it it natural to talk in point, inches because your A4/A3 sheet of paper doesn't suddenly change to A5/A6, so you're all-ways designing for a fixed size.
Most high DPI web-browser/software run into the main problem that the world runs on <= 96dpi screens.
I'm typing this on a 27″ iMac 5K with 218 dpi resolution. My smartphone (an iPhone 6) has 325 dpi; my 9.7″ iPad has 264 dpi. And you don't even have to use "retina" high-DPI displays to beat 96 dpi. The older 27″ displays, with 2560×1440 resolution, have 109 dpi when you work out the math; the MacBook Air 11″ has 135 dpi.
I think it's fair to say the world ran, past tense, on screens that were 96 dpi and under. But that's just not a very good assumption to make anymore.
There is a A LOT of content being built, have been build by designers for 96dpi. The reason why it isn't a concern for you is because that standard exists, and your webkit knows you're on a 218 dpi monitor and scales to the px unit.
Consider it this way. When you had a A4/A3 size paper it it natural to talk in point, inches because your A4/A3 sheet of paper doesn't suddenly change to A5/A6, so you're all-ways designing for a fixed size.