A scant 1/4" is just a thread or two short of a full size 1/4", to accomodate what is lost when the seam is folded to one side, with the thread thickness inside. If you sew a full quarter inch seam, you'll lose a bit more in that folded seam with the thread inside, and your block will finish smaller than it should. If you sew the scant quarter inch seam, then what is lost is lost in the seam allowance, not in the remainder of the piece. Make sense? It may sound insignificant to be concerned about a thread width, but it isn't. If you are off just a 32nd of an inch, and repeat that 32 times over the width or length of a quilt top, your top is now a full inch off. If your quilt top was constructed of only equal sized squares all over, it wouldn't matter in matching up the seams as long as your seam allowance size was consistent--it would just mean your quilt top would not end up exactly the expected size. But for any other pattern, involving pieces which are not squares, (triangles, rectangles, etc.) and with unequal numbers of seams vertically and horizontally within individual blocks and over the entire quilt top, all those minor inaccuracies add up and make seam matching impossible or just very difficult.