I always thought that they make them that way because full inches and half inches are the most common measurements. For squaring up a block, there's 1/4" seam allowance on both sides - if your finished block is 8", then your unfinished block will be squared up to 8 1/2". I guess it depends if you're measuring from the middle of the block or the whole block. Some blocks don't have a middle seam to measure from.