using css piex calaculating screen size, where is wrong?

In css w3(here),

1px = 0.75pt, 1pt = 2.54/72 cm,so 1 css pixel:1px = 0.26mm

for example, the CSS pixel of iPhone4 is 320px * 480px(not device pixel),
so in width ,the length should be:

320 *0.26 = 83.2mm

but the iPhone4 is 3.5"，so the length in width is about 60mm,

it dont match the result i calculated above
where is wrong?
thanks