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.
column-end means really "end+1", why?
problem: when you go from left-to-right: column-start-3,column-end-5: means column #3 is included
when you go from right-to-left: column-start-3,column-end-1: means column #3 is NOT included
So if you read "column-start-3" you don't know if column#3 is included or not, it depends on column-end.
"column-start-3,column-end-5" should include column 3, 4 AND 5
This is from level 10:
>For example, grid-column: 2 / 4; will set the grid item to start on the 2nd vertical grid line and end on the 4th grid line.
What's the correct interpretation of "start on 2nd and end on the 4th"? shouldn't the 4th be included?