Well,a scant 1/4 is to accomidate the thickness of the fabric and thread after the pieces are sewed together.Try cutting a 1 1/2 in,square and sewing with an exact 1/4 in seam.Now re-measure,do you have a 1in. finished square.No,because the fold of the fabric and the thickness of the thread takes away from the final finished measuremant.So if you sew a scant 1/4 in. it's more exact after the quilt is pieced.But,I also just do a really good job and then move on.