Not sure this would be the reason, however you might check on it. Sometimes when I am sewing a straight seam, and it comes to the end of the strip, I tend to make the seam allowance a little smaller than I did at the beginning of the strip. That would tend to make it have a little wave in it. I have to really be careful and make sure the seam allowance is the same. I love the color selection. Your Pokey quilt for GS is so cute. Love the backing. You do good work. Merry Christmas.