The scant 1/4" is all about turn-of-the-cloth. When you iron a seam, some fabric is taken up by the turn at the seam. If you start with two 5" squares, sew an exact 1/4" seam (meaning 1/4" allowance to the right of the sewing thread), and iron them open, theoretically you should have a rectangle that measures 5" wide by 9.5" long. However, in real life it will measure something like 5" wide by 9.46" long. This is because part of the fabric not in the seam allowance has been taken up by the turn at the seam line.
With many patterns, the exact finished size doesn't matter.
With some patterns, it does matter. For this reason, it is good to be able to achieve a scant seam when you need it.
The easiest way for me to ensure even scant seams is to create a physical barrier on the bed of my machine. I use molefoam for this. Basically all I have to do is butt the raw edges up against this barrier to sew a consistent seam at the depth I want.