In my mind, the "scant" quarter inch is about one thread less than an " actual" quarter inch.
I think the reason the instructions say " scant" is so that the seam is no more than a quater inch.
For some of us that used to sew garments that called for a 5/8 inch seam allowance, it was a major adjustment to sew that skimpy 1/4 inch seam.