This is what it says in step 4: Your rows will be a little off on the ends and will need to be trimmed after all sewn together. Press all rows.
Since the 2 striped pieces in one row are 14" (resulting in 27" after all seam allowances) and the other row has 1 stripe of 27" (resulting in 26.5" after all seam allowances) these rows should be 1/2" off when sewn together. I don't see why they couldn't have given the proper dimensions (e.g., make the 27" piece half an inch bigger) in the first place.