You're not subtracting the seam allowances. Every time you sew two strips together, 1/2 inch gets subtracted from the total of the 11". Sewing four strips together will subtract 1/2" three times or 1 1/2 inches. 11 minus 1 1/2 equals 9 1/2. Then when the blocks are joined to each other, an additional seam allowance is taken off both sides, equally a 9 inch finished block.