There really wasn't a specific reason for the naming convention. Although adding "col-" to all of the columns allowed me to easily select every column using [class*='col-'].
I had the same problem as it seems the convention isn't explained on the website. However as soon as I took a look at the code it became clear that the numbers represent fractions.
.col-2-12 is a column that is 2/12 in width (same as 1/6) and .col-10-12 is 10/12, respectively.