Understanding CSS Grid Systems from the Ground Up

How do your column width calculations arrive at a different point than frameworks like Foundation and Bootstrap. What I mean is, why do they evenly divide the width (100%/12) while you do not? Eh, I mean what benefits does that bring over Foundation/Bootstraps way? What makes your way better than theirs?

I’m just trying to follow along. I understand your calculations but I’m just curious why you decided to not follow that evenly divided route. Is this just a case of ToMAtoe Tomatoe?