Hi,
I always piece with a scant 1/4 inch seam.
I have the Babylock Symphony....the quilt piecing stitch for 1/4 inch is set at 5.5 on it, that's a true 1/4 inch.
I piece at a stitch setting of 6.0 which is a scant 1/4 inch.
When you use a true 1/4 inch and press your seam to one side you are losing that "one thread width" as Eleanor Burns says, so your stitch is now 1/4" plus that tiny width.
If you sew with the scant 1/4" and then press...it's a true 1/4" when done.
That's how it was explained to me.......not so much the sewing of the seam, but the pressing it over is where you lose. Make sense? Hard to explain....
Hope that helps.
Bev