Be sure to set the ruler on edge and squat down and make sure you don't see air between the ruler and the table. One of mine was giving me fits and I discovered it had a "whoo-hoo" in it (my husband's term for when wood has a curve). That will make a difference...also, your sewing makes an even bigger difference. Make sure ALL your seams, on both sides are 1/4".
also - this may have been mentioned.....double check the measurements on the ruler with another ruler. My old HUGE cutting board (one of the originals) is off by 1/8" on one end...!!! Don't ask me how many years it took me to figure that one out!!!