I think I see a slight bow in the upper middle of your block. If everything measured the same to that point, something has stretched, probably with a little too aggresive pressing. Try pinning your block to the ironing board {or another flat surface) in the correct measurement, spray with water, and let dry. I'll bet it will come out right.
The other thing to check is when you are cutting accross a lot of seams--like this one. Make sure that the weight of your hand is on the area you are cutting--even if it means that you have to stop a few times on the way down the cut and remeasure and reposition the ruler. The seams will throw off the cut if the pressure is not the same.
Honestly, don't worry about a small difference here and there. Such a small amount will work itself into the quilt quite easily.
Last edited by GingerK; 03-10-2012 at 08:38 PM.