NOT an optical illusion! Well spotted! :) I use a short stitch (2 mm) to avoid puckers - you get fewer with a short stitch - and a .5 mm zigzag for stretch. I also pull the seam while sewing, to get the maximum stretch in there, giving the seam the same amount of give as the bias fabric. This wouldn't be necessary if you serged the seam under tension or used a differential feed.