Originally Posted by Krisb

If you place the square in the center, you sew two seams in each direction. The two seam allowances take up 1" of fabric, so the block reduces to 9" (8 1/2 finished). If you place the square in a corner, you only do one seam each direction, so you end at 9 1/2" (8" finished). You're not doing anything wrong, that's just how it works. Another vote here from trimming them all to 9" before sashing.