Do you measure at the stitch line or after? Do you steam shrink your block if it is slightly larger? Do you find different brands, weight (50 or 60) thread makes a difference whether you use a 1/4" or scant 1/4"?
I use good quality serging thread which creates less bunk and the result is a perfect 1/4" seam with the Bernina #57 foot. However, when I use regular 50 weight thread I use a scant 1/4" which is accomplished by moving the needle one notch to the right using the #57 Bernina foot.