That because the measurements used in web browser, “px”, doesn’t mean “screen pixels”.
According to the W3 website :
“The px unit is defined to be small but visible, and such that a horizontal 1px wide line can be displayed with sharp edges (no anti-aliasing). What is sharp, small and visible depends on the device and the way it is used: do you hold it close to your eyes, like a mobile phone, at arms length, like a computer monitor, or somewhere in between, like an e-book reader? The px is thus not defined as a constant length, but as something that depends on the type of device and its typical use.”
So a very high definition smartphone can be seen as being less than 800(px)x600(px), depending on its screen size and aspect ratio, while still being 4k or more.