I sometimes get these discrepancies, too, and I have a very old machine. Is it better one way than the other (strip on bottom, strip on top)? When the strip ends up too long, is it because it has slipped on the starting end? Or does it actually appear to be stretching in length? How are you pinning? I usually put a pin perpendicular to the edge near the end point of the seam, and parallel to the edge along the length of the seam.