I agree with Tartan that the light fabric may not be as tightly woven as the other fabrics. Before I cut long strips I always check to see how they stretch. Most patterns tell you to cut strips width of fabric because they should not stretch as much as length of fabric. I have found some fabrics are almost as stretchy width of fabric as length of fabric.
Be sure to PRESS, not IRON your blocks when blocking them. That helps eliminate the fabric curves. Also check you pressing surface. It needs to be firm so that the fabrics do stretch when pressing.