The reason you can start with a 10" and end up with a 12.5" block is because you are starting with TWO 10" blocks one on top of the other. After sewing them together and cutting you open them up which gives you the full size of both of the blocks minus your seam allowances. In reality you're starting out with the equivalent of a piece of fabric that is 10"x20" and shrinking it down through cutting and seam allowances to a 12.5" square.