Make a blank page. Insert a section, put a container in that, and then a grid with a 100% width. Delete the second grid row and increase the number of grid columns to 4. If you put a div with a background color into each cell you will notice that the total width of the grid is only 4/5 of the width of the container. Add a 5th column and it takes up the full width again.
Why does this happen? Is this by design? How can I get a grid to take up the full width?