The discrepancy in the size after sewing lies in the measurement between the needle and the edge of the fabric. Although I have a 1/4" foot on my machine, the edge does not line up with the 1/4" marking on the machine itself. Therefore, my blocks are a tad off also. They are more than 6-1/4" but less than 6-1/2", so I guess the scant 1/4" will need to come into play especially on the machine I'm using. I could sit down and take out all the seams from the 12 nine-patches and resew, but I don't think it will matter that much in the long run.