Sometimes it is the amount taken up by last long seams. Due to the bulk of fabric at intersecting seams, the seam allowance needs to be adjusted. I cannot tell from the picture if the short width corresponds to the long seams that join the rows.
When I did the first 18 inch Civil War Tribute block, I was half an inch off. Told to use a thinner thread and a scant 1/4 inch. The second time, it came out fine. It is always a challenge to me.