Why absolute units in CSS are not shown really absolute in different screens?

The so-called absolute units (cm, mm, in, pt and pc) mean the same in CSS as everywhere else, but only if your output device has a high enough resolution.

source: W3C

when I put the preceding code in my project I expect to see 1cm everywhere...

.myColumn{
   width: 1cm;
}

W3C says for low-resolution screens, case would not be the same.

Why?

Why cannot they produce 1cm correctly?

In cell-phones, and tablets and... the only screen unit is pixel.