Why does css write down the height of different ppi phones? why are the heights of different phones so high?

for example, the iphone6 and iphone6plus,dpr are 2 and 3, respectively, and the PPI is 326 and 401, respectively. Suppose I html directly write dead style="height:100px", to see the effect is the same height. But what I can"t figure out is that the actual size calculation process is not 100 times 2max 326, 100 times 3ax 401, obviously different, ah, why is it equal height? The study found that it seems to have something to do with setting ideal viewport. If ideal viewport, is removed, it is not equal to height. Would you please explain?

Mar.13,2021
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