Shift Up and then Shift Down does not give the same color

[tmrosen]

Description

Shouldn't shifting up one color and then shifting down one color give the original color?

Steps to reproduce

In CSS
border-bottom: 1px solid #d6d5d5;
Shift Up
border-bottom: 1px solid #d8d7d7;
Shift Down
border-bottom: 1px solid #cdcccc;

Another example
outline: 1px solid #26b050;
If you shift up to very light and then down to dark you never get back to green. It is just shades of grey.

Expected behavior: Shift up and then down should give the original hex color

Actual behavior: The color looks similar but the hex is different.