Not just the rulers, but the guides on the sewing machine can also throw off a measurement. Also, when you iron seams to one side it takes up a tiny bit of fabric for it to go over the fold. I think you need to find out where on your ruler you need to place the fabric to get the exact size you need. Then do the same on your machine. On my sewing machine, I sitch a "scant" 1/4" seam to account for that extra fabric taken up by the fold. By "scant" I mean I place my fabric so it lines up on the needle side of the mark instead of right on top of the mark.