Plus there is the issue of measurement. Are you using the inside, outside or middle of the line on the ruler. I'm not good enough to feed my material under the needle perfectly straight. There always seems to be even a tiny bit of waver where I lost my focus or went too fast or blinked. This tiny bit of movement affects the depth of the seam allowance so I just do the best I can and most of my stuff matches up quite well. I'm careful but not fussy.