Because it ensures a page is useable on a mobile phone, even if the author didn’t take mobile into account: the screen can accommodate for about as much information as a regular 320px computer screen. If you try to use every pixel as you would on a regular screen it would be illegible for most users.

> Explain why pixels on smart phones are not physical pixels, as I would expect them to be. Why does a phone need to act differently, kind of dishonest about its pixels?

The phone is not acting differently. The phone is acting according to the specification, which is that a pixel is 1/96th of an inch. This happens on all devices, not just phones.

From the spec:

> The absolute length units are fixed in relation to each other and anchored to some physical measurement. They are mainly useful when the output environment is known. The absolute units consist of the physical units (in, cm, mm, pt, pc, Q) and the visual angle unit (px):

> 1in = 2.54cm = 96px

https://developer.mozilla.org/en-US/docs/Glossary/CSS_pixel

https://drafts.csswg.org/css-values-3/#absolute-lengths