The columns do in fact add up to a width of 100% when combined with the percentage-based left margin for each column except the first one to form the gutters. Hence the complicated calculations.
The difference with Bootstrap is that it sets a zero margin on the columns, and instead sets a padding on each column to form the gutters. This simplifies the calculations with (100%/12) as you noted.
They are two different approaches for creating gutters with slight differences. It’s merely a matter of preference.
Hope this helps.