Ok, I just went out to Red Heart and had a look at the pattern. It is important where you connect each of the colors in the border because the green sets up where the pink shells work in and the pink shells, being so large, are almost like a fan and they must begin and end in just the right place around the edge. It would not do at all if the shell wound up being too close to the corner. There wouldn't be room for you to finish the shell before the corner stitches would have to be made. I suppose it would be possible to attach at any other place around that row, providing you were able to stay true to the pattern, but it would appear that this is the only place in that row that the designer has calculated for the pattern to work.
Mark