Sounds like you have been visited by the "scant" ghost. the sizes you cite "should" give you the 12.5 block, the finished size of the center is 4, then add two 2inch dark and two 2 inch light for a total of 12 inches with the outside seam allowance still in position for a total of 12.5 inches. Try re-sewing the block with a seam allowance of slightly less than the quarter inch you are typically using. That's called a "scant" quarter inch. Measure each step to ensure you are staying true to the correct size. Are you using a quarter inch foot? If so, keep the edge of the fabric just shy of the quarter inch mark on the foot so you know you are "scant". I wouldn't add to the center, just try to resize my seams. Hope that helps!