Confusing: Using units versus NO units for widths??

[moonbeetle]

In grid.scss

// Defaults which you can freely override
$column-width: 60px;
$gutter-width: 20px;
$columns: 12;

In style.scss

// Specify the number of columns and set column and gutter widths
$columns: 12;
$column-width: 60;
$gutter-width: 20;
// Remove the definition below for a pixel-based layout
$total-width: 100%;

Using units versus NO units for widths??

This is confusing, please explain, I know that it is working but I want to know WHY it has to be that way.