I believe it's talking about the vertical lines, not the column space between them. So the "+1" is because it's talking about the rightmost edge of what you're

I believe it's talking about the vertical lines, not the column space between them. So the "+1" is because it's talking about the rightmost edge of what you're calling a "column". here's an image that may explain better. I numbered the vertical edges, start is green, end is red. https://imgur.com/a/xCplwAt https://imgur.com/a/xCplwAt