Originally Posted by np3

The fabric strips are positioned right sides together, paying close attention to keep the raw edges aligned at the left edge. As the strips are sewn together, care is taken to keep the left edges alligned with the tape strip. If there is any variation in strip width, it will fall within the seam allowance.

This is what you are missing. You don't go in and out with the left side. You keep a steady pace as you sew and any small variation on the left side will be taken care of when you add the next strip. Eventually the left side becomes a right side.