When using shorthand for margin (or padding) with only two values you are not setting a X/Y position - you are setting four margins, using the same value for top & bottom (vertical margins), as well as right & left (horizontal margins). You can also pass four values and they will start with margin-top and continue clockwise around the box (top -> right -> bottom -> left).
I usually remember this using the word "trouble" without any vowels (TRBL).
Anyway: for positioning there is only two values, and it is common practice to use the vertical axis (x-axis, 0 is top) and then then horizontal (y-axis, 0 is left), so using a negative value for the y-axis on background-position would move a background the same direction you would move the box if you were to give it a negative left margin.
.class1 {
background-position: -20px 0; // move background 20px left
margin-left: -20px; // move box 20px left (margin, following items will also move)
}
.class2 {
position: relative;
left: -20px; // move box 20px left (position, following items will stay put)
}
So I guess what I'm trying to say is that the values are basically coherent, depending on how you look at it ;)