The reason a scant 1/4 inch is used is because there is fabric width "lost" to the seam itself - just a few threads' width, but enough to make a size difference after the seam is sewn. A scant 1/4" allows the finished piecing to match the desired dimensions. If a full 1/4 inch seam were to be made, the overall blocks would be smaller, so all pieces would have to be sized up some tiny amount in order for the finished sub-units and blocks to be the desired size.
Think about 10 2x2 squares - you start with 2 1/2 inch squares, seam them in a row using a scant 1/4", and get a finished strip that's 20 1/2 inches long. That's easy to join to another 20 1/2 inch solid piece of fabric.
Now take those same squares and sew them with a full 1/4" seam. The finished strip might only be 20 1/8" to 20 1/4". Now you go to join to the solid piece of fabric that's 20 1/2 inch and guess what? There's a size mismatch.
So what's a pattern designer to do? They'd have to calculate all the different cut sizes, which are going to be weird measurements, not even increments of 1/4, 1/2, 1 inch, etc. You're always going to have that slight loss of width to each seam, so rather than work with weird cut measurements it's easy to compensate for the loss by adjusting the seam width.
Here's a link to a page that shows the width loss
Scant 1/4" seam
And here's
another with photographs that show the issue